Scott Barton's
98 M Roadster Stereo System Upgrade

(aka Z3 Stereo Installation for Dummies)
 After getting my M Roadster and driving it around for a while, I couldn’t believe how poor the sound system was, especially for an “upgraded Harmon Kardon” system.  The system was useless with the top down.  I pretty much drove around with the stereo off and used the exhaust note as my listening pleasure.  After a while though, I needed to listen to some music.  I probably could have dealt with the poor sound system if it wasn’t for the crappy tape deck.  I couldn’t believe that a $45,000 car comes only with a tape deck?  I know I could have bought the BMW CD changer, but I wasn’t about to pay $500 for it, especially since it would only work with the entire factory system in place, and for a few hundred more I could have a real nice system that I would be able to hear with the top down.  So that’s what I set out to do.

My constraints were:
  1. CD/MP3 player
  2. Be able to listen to the music w/ top down at 80+mph
  3. Must retain a factory/stealth appearance
  4. No loss of cabin or trunk space
  5. Cost less than $1000
Here is the list of components I decided on:  
The install process:

My car was in storage for the winter, and I was not going to put it on the road until late April.  I took a little over a month to do all of the research of what I was going to do, which components to buy, and find the cheapest places to buy them.  After that, I had almost a month’s time to do the actual install.

The first thing I did was to remove all of the carpet from the trunk.  It requires a special tool to easily remove all of the black plastic caps.  I just used a set of bent needle nose pliers.  I also pulled back, but did not entirely remove the carpet in the area where the convertible top goes down.  On the driver’s side, near the strut mount, you will see the rubber grommet where the wires go from the trunk into the cabin.

I then removed the stock HK amp.  To get the amp out, you pull out the small black connector, then the big one next to it has a lever lock.  There are little tabs under the white lock that you have to press down to release it, then pull the lever, and then out comes the connector.  The third one I had some trouble with.  It just pulls out, but I had a very hard time getting it out.  This one required a lot of force to remove.  I think I just grabbed it with pliers and pulled hard until it came off. 
I removed the stock subwoofer (following this removal process) so I could gain access to that area in order to run my wires later.  The connector is kind of tricky.  It is easier if you separate the plastic connector from the metal bracket first, and then separate the connector itself.   Also remove the subwoofer speaker grill by lifting from the bottom.

Next up was to remove the center console.  Remove the center storage compartment first. Then remove the 5 screws on each side of the center console and the 1 in the middle under where the storage compartment was.  I couldn’t actually remove the whole center console, but it now was able to pivot up at the top near the head unit.  This was enough to be able to run my wires/cables from the back to the front.

I then removed the stock head unit following these removal instructions.  The connector on the back of the head unit is kind of tricky.  There is a tab on it that you pull up on and it will force the connector to separate from the head unit.  It is tough to lift it up all the way, but just keep pulling.

I then removed the plastic kick panel covers.  The first thing I did was remove the covers that are actually above where your feet would be.  They just have 2 or 3 plastic screws holding them in.  Turn the screws turn and they should come out.  Then just wiggle the covers free.  I then removed the actual kick panel covers using these removal instructions.  This is a bit tricky since there is a tab on the cover that slides underneath the doorsill cover.  You need to gently flex the kick panel cover in order for the tab to come out of the doorsill.  Be very careful when removing and installing these covers as they will break.  I broke mine and had to epoxy it back together.  If there is an easier way to remove the kick panel covers, or to remove the doorsill covers, let me know. 

I also removed the stock 5.25” speakers at this time.

Now it’s time to actually run the RCA cables and speaker wires.  I only needed to run one set of RCA cables since I was only running the BA 6.5 kick panels and the sub off of the Amp (which was bridged 3-way).   The subs get their input from the same RCA inputs as the other speakers on my amp.  I would have liked a separate sub input, since my head unit has RCA outputs for that, and has the ability to fine tune the subwoofer output.  Since I am running the rear 4” coax’s off of the head unit (60W) I had to run new speaker wire from the head unit to the rear speaker locations.

I heard that running new speaker wire to the kick panel speakers was a PITA.  I also heard that since you were only using one set of tweeter wires, that you could cut the other set of tweeter wires and splice them into the current kick panel speaker wires to effectively double the stock speaker wire size for the kick panel speakers.  This is what I did, but I now feel that it is not good enough and that the BA 6.5’s are not getting the power they need.  It is bad enough that I am considering running new speaker wire to the kick panels.  (I will cover how I actually re-used the factory wire later).


To run wires from the trunk to the cabin you need to go through the rubber grommet on the driver’s side near the strut tower.  I could not actually remove the grommet, but you should be able to push a portion of it up from inside the trunk.  I used a screwdriver to push it.  This will allow you to push your wires through the grommet.  Once you have pulled all of your wires all the way through, you should push the piece of the grommet back down from the top, from the inside of the cabin.


Once you have your wires inside the convertible top area, you can run them into where the sub area is.  There are 3 screws under plastic tabs that you can remove to help you slide the wires under the plastic piece.  Pull the wires all the way out to where the subwoofer speaker grill was.  Now run the wires under the center console.  In order to run them all the way to the front, I ran them under the center console and then out the side on the passenger side.  Then I slid them back up in front and to the opening for the head unit.  Then to get the wires to be in the middle of the console, I grabbed one end of the wires in the head unit opening, and grabbed the other end by the subwoofer speaker grill area and pulled them tight.  This brought the wires back up under the console and up into the middle.  Now arrange the wires so that they are out of the way when you put the storage unit back in and the subwoofer back in.  I also tucked them out of the way so that when you open the subwoofer speaker grill, you can’t see them. 

Then I installed the Head Unit following these instructions as a guide.  Make absolutely certain you wire the harness up correctly.  Here are Crutchfield’s instructions page 1, page 2, page 3.  I also used these instructions for installing the Alpine 7894 head unit (these instructions are useful for all HU’s).  I have heard from many people that the factory power wire is not enough, and that you have to run a dedicated power wire to the Alpine 7894 head unit (the instructions with the unit also recommend this).  The stereo shop I went to get the power wire from told me that I didn’t need to run the power wire, and that they have actually installed that head unit using the factory wire w/o problems.    Since I am only running the rear 4” coaxials off of the head unit, I decided not to run the power wire and just re-use the factory one.  I have had no problems so far.  In fact the rear 4” sound great.  There are 2 nuts back and to the right, inside the area of where the head unit goes.  Either one of these seem to work well for a place to ground the head unit.  

Now let’s install the amp and Boston Acoustic crossovers.  After seeing a pic of Carl B’s amp install, I decided that would be a great place to install my amp.  The only problem is the size constraint (10”x11”x2” or something like that).  This size constraint was the determining factor in purchasing the Alpine F320 5/4/3 way amp.  Bridged 3 ways it says the power output is 100Wx2+120Wx1 RMS.  I actually believe that this amp is a little underpowered to drive the JL Audio sub and the Boston Acoustic 6.5 components.  It could just be that the factory wires (even doubled up) are not enough for the BA 6.5” woofer component.  Re-using the factory wire for the BA tweeter component is fine.  In fact the tweeters sound great and even overpowers the rest of the system some times.  I used this wiring diagram to figure out which wires to re-use and which ones to cut and splice to use with the BA 6.5 kick panel speakers.  I mounted the BA crossovers in the stock location of the HK amp.  I reused the HK amp mounting bracket and made a custom Plexiglas plate to mount the crossovers.  I attached the crossovers to the Plexiglas with industrial Velcro.  Now just wire up the amp and crossovers per manufacturers instructions.

OK, now let’s install the speakers.  I started with the kick panel speakers, since I already had the kick panel covers off.  I used these instructions [2] to help me with the kick panel install.  These instructions cover the quirks of installing the new BA 6.5’s.  Boston Acoustics 6.5” driver requires some dremeling of the sheet metal.  Make sure you use a vacuum when you do any cutting/dremeling to pick up any metal/plastic shavings.   The speakers will come in contact with the plastic kick panel that installs over the speaker. You have to sand down the protruding plastic ring on the backside of the kickpanel.  I also cut some of the foam padding to make certain there was room.  I still think that the speaker may be hitting the kick panel cover.  Mounting the speaker on the outside of the kick panel cover would probably have better acoustic results, but I wanted to remain stock looking.  You should stuff some Polyfill in the door panel.

Now we have to install the tweeters.  I used Ron Stygar’s instructions along with these tips on removing the door panels.  The picture with measurements to the location of the plastic snaps helped tremendously.  I used a small screwdriver wrapped in electrical tape to remove the door panels.  A plastic screwdriver would have been great.  Removing and installing the door panel is actually not that hard, and I can now do it in about 5 minutes.

Most people recommend that you put the BA 1” tweeter in the factory 2” tweeter location.  I ended up putting mine in the factory 1” location because they fit better.  I cut the factory tweeter connector and attached it to my tweeter to make it easy to connect/disconnect the tweeters.  I used a piece of metal strapping with some foam tape on the back to hold the tweeter in

Use these tips on re-installing the door panels.

Next I replaced the rear speakers.  I followed these instructions on replacing the stock 3” speakers with 4” coaxial speakers.  These instructions help for replacing the stock 4” speakers on newer Z3’s.  I found it hard to get the speakers out.  A threaded collar holds them in.  On the collar are knurls.  If you place a long screwdriver on one of the knurls and hold it there with one hand and pound on the top of the screwdriver with the palm of your other hand they will eventually loosen.  Once they are loose, they are easy to remove.  You will have to cut a hole in the plastic to make room for the 4” speakers.  Again, when cutting the plastic, make sure you use a vacuum to suck up all of the pieces of plastic and dust.   Install some Polyfill in here as well.  When installing my 4” speakers I removed the little Infinity logo from the grill, as I did not want any visual clues that there was an aftermarket system in the car.

I was able to be a tester for the Z3 Bass sub enclosure.  Kevin’s subwoofer comes with very easy instructions, which include using the instructions for the Sunfire sub.  The only thing not mentioned for the HK system is that you have to remove the current subwoofer brackets before installing the sub. 

Info on my stereo system components:


Alpine CDA-7894 Head Unit
 I chose this CD/MP3 player because the display can be amber, which matches the stock BMW lighting.  It has a very powerful 60W (27W RMS) internal amp.  It also has lots of cool powerful features.  I am using it to power the rear 4” coaxial speakers only.  I really like this head unit, and it works well powering the rear speakers.  I could not get the dim on the head unit to work though.

Alpine MRP-F320 5/4/3 way Amp
 My main constraint on the amp was the size.  This was one of the most powerful amps that would fit in the location I wanted.  It’s power rating is 40Wx4 + 120Wx1 RMS.  I am using it bridged 3-way which has 100Wx2 + 120Wx1 RMS.  It powers the BA components and the subwoofer.  I’m not sure the amp is powerful enough for the BA’s and the sub.  If I turn the volume up over 20, the BA 6.5” woofer component and the sub distort and get muddy.  This could be in part because I re-used the stock wires instead of running new ones.  It does not have a seperate subwoofer input. Instead, it uses the inputs for the front and rear speakers as the input for the subwoofer as well.  I do not like this because my head unit has a subwoofer output that would allow me to fine tune the subwoofer, but because there is no sub input on the amp, I can’t use those features.

Boston Acoustics Pro 6.5 6.5” components
I went with the Boston Acoustic 6.5’s since most people recommended them.  I heard they work very well with the odd acoustics of the roadster.  The tweeters sound nice, but I am disappointed with the mid-low level of the woofer component.  I think that they need new wires run instead of re-using stock ones.  They may also need more power.

Infinity Reference REF452I 4” coaxial
Some people suggest either not upgrading the rears, or just getting cheap coaxials.   I found that the rears do contribute a lot.  It is important for them to have a wide frequency response, especially in the mid-lower end.  They do not need to have crystal clear highs since they fire into the back of the seat, but the mid-low range is important.  I chose the Infinity Ref 452I 4” coaxials because the featured a Plus One design which allows the speaker to actually be larger than most 4” speakers.  They also have wide frequency response, and can handle a wide range of power.  Using the head unit to power the just rear speakers works very well.  I am very impressed with the sound quality of these rear speakers powered from the head unit.  In fact the BA 6.5 woofer components, and the subwoofer get muddy before the rear 4” speakers or the tweeters distort. subwoofer enclosure with JL Audio 6w0 6.5” sub
I was looking to use Carl B’s custom subwoofer enclosure.  But before Carl posted his instructions, Kevin contacted me about testing his subwoofer.  They both use the JL 6w0 sub.  Carl’s is a sealed box, whereas Kevin’s is a ported box.  Kevin’s box really compliments the system and fills in where the BA’s are lacking, but it doesn't give you that low, powerful bass you get with a real 10" or 12" sub (wishful thinking I guess).  It is a tremendous improvement over stock though.  On a scale of 1-10, the stock sub would be a 2 and the Z3Bass would be a 7.  Kevin’s has detailed instructions, and the install was very easy.  If you plan on using the stock subwoofer location for your sub, and don’t want to make an enclosure yourself, I would highly recommend that you get the subwoofer enclosure.


Essential Z3 Stereo reading – Info on stock HK system (Robert’s Stereo Upgrade to Learn From)

Ideas on stereo upgrades – (*ideas on what to take into consideration) (DC's Stereo Upgrade) (1.9 Non HK Stereo Upgrade w/BA 6.4’s) (Kevin’s Stereo Upgrade) (Sound System Upgrade) (Skyhogg’s) (TexZan Stereo Upgrade) (Big Book of Car Audio)

Now that you’ve read up on the stock system and possible upgrades, and have now decided on what you want to get, let’s install your new system:

Removing stock head unit (Skin’s CD Skip Fix)

Overview of installing new head unit (Radio Install Guide)

Installing Alpine 7894 Head Unit (good info for all HU’s) (7894 Install)

Running wires from trunk

HK Amp wiring guide

Neat place to mount amps

Removing and installing kick panel speakers (MB Quart Replacement Speakers)

Tips on Installing Boston Acoustic 6.5’s

Removing Door Panel

Installing Door Panel

Replacing Rear 3” speaker with 4” speakers

Removing/Replacing stock Rear 4” speakers

Removing stock sub

Stock subwoofer rattle fix

Kevin’s Custom Sub Enclosure (

Carl B’s Custom Sub Enclosure (DIY)

Part Number for plastic dash trim screw head caps

Click here to download a Word version of this document.

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