Adding 6x9 speakers & Subwoofer to a BMW 635

Adding 6x9 spearkers and a subwoofer to a BMW 635

By Peter Siposs - March 15, 2002, Updated April 4th - SEE BOTTOM

So you're not so happy with the sound coming from the existing stock speakers. The odd size of the rear speakers makes you wonder about replacing them with something modern. You want to make it look original though so what to do? You can indeed fit 6x9 speakers and a subwoofer plus amplifier and nobody will be able to see anything different when you're done.

How it looks on my car before and after:

The original rear speakers on my 89 635 were only 4 or 5 inch round with a seperate 1 inch tweeter all mounted in a rectangular plastic housing. Pretty feeble for these cars. Fitting anything larger than 6x9 would requiring loosing the original grille cover and I wanted to retain the original look. I found that a large magnet 6x9 coaxial speaker would just fit in the shallow depths of the original area. Mounting them would require an adapter plate and some slight modifications.

To access them, remove 2 screws holding the bottom corner of each seat and pull them forward. Now the seat is out of the way and you can see how the pods are mounted. Undo two 13 or 14mm nuts holding the pods down. Lift upward to disconnect the wires and the rear of the pod just hooks in and the whole thing can be removed from the car. Unscrew the original speaker mounting base and remove.

Here is how the original speakers looked:

Here is the area as viewed without the pod attached. You can see how shallow it is:

Here is the pod without a speaker:

Its made of steel and covered with vinyl and foam underside. Trim away the opening a little with a utility knife. Also flatten the underside by cutting away high spots of foam. The mounting screw holes are indented and I flattened them in a vice. This ensures the new adapter plate fits flat and snug. I made a cardboard template first to find the right sizing including the size of the new speaker. Due to creases in the steel, I formed the plate to fit close. Once I got the shape I liked I cut the plate from thin masonite with a jig saw. Its about 3/16th thick I believe. Basically the same stuff used for pegboard without the holes. I drilled out the original speaker mounting holes to allow screwing from the top downward. After applying a silicone adhesive I screwed the unit to the pod. The silicone prevents rattling and provides a solid transfer of resonance.

Here is the pod on its back with the plate attached:

Same as viewed from the top:

Attach the new speaker with screws. Be sure to use washers and lock washers to prevent having them come loose or pulling through.

Here is the speaker as viewed without the cover in place.

Do the other side the same way and reinstall the pods and the seats. Don't forget to plug the wires back in!

Problem areas to consider for mounting additional speakers:

Adding a large subwoofer to the center of the trunk ceiling would require relocating the fuel vapor cansiter as shown here:

Adding speakers to the vertical wall below the rear seats on models with rear AC would require major rerouting of freon hoses and related equipment as show here in the photo below. Also, for cars without AC, the wall under the seat where some people mount 6x9 or 5" speakers is NOT big enough for an 8" sub. The wall is about 7-1/2" high or less. The seat may foul the back of the speaker anyway.


In an effort to enhance the bass notes even further I added a subwoofer plus amplifier and put two holes under each 6x9 speaker. This significantly improved the bass sound and finally obtained the sound I was seeking without any visual changes to the car or valuable space taken or high expense.

First I'll discuss the addition of holes under the 6x9s previously added. The cavity under the speakers are very shallow and do not promote decent bass. Most cars have a direct opening under the speakers to the trunk but on this model they are blocked. There are effectively 3 layers under the speakers. A thin compressed wood area, steel deck area, and the felt ceiling of the trunk. Once I gained access to the area and removed the trunk ceiling, I used a door lock hole saw to cut 2 circles through the compressed wood, then an electrical conduit knock-out punch (aka chassis punch) to cut through the steel deck area under it. Another method would be to drill numerous 1/8" holes all the way around very close together, then use a stepped drill bit which will enlarge the holes so they touch the other opening and you have a hole, though not very smooth but it works. Highly suggest using a center punch first to help the bits from wandering. Trace a paint can or roll of electrical tape to make the circle. An air nibbler or other cutter could be used too but I bought the knock out punch kit on ebay for this and other uses and it works really nicely leaving a distortion free steel hole and no debris.

These are the cutters used:

Here are the resulting holes as viewed from the interior:

..and from the the trunk (with felt ceiling panel removed):

To really obtain a significant improvement of bass however, I added a subwoofer once I determined I could mount one without having it seen or take up any trunk space. My car has rear seat AC/cooler which has an unused cavity directly behind the cooler area and under the center pad. 6-series cars without rear AC also have room here but not certain it is large enough to house something like I used. I was going to make my own speaker box but decided to simplify by buying something off the shelf. Its originally intended for a 6x9 speaker, but I cut the hole larger to fit an 8 inch subwoofer. A box intended for an 8 inch speaker was too big for the area, as was using a 10 inch speaker because of the depth available. Having it facing the rear of the AC isn't ideal, but it resonates nicely and surely has to be better than mounting something in the trunk.

Here is the area once the seats were removed. Notice the area directly behind the cooler area.:

Four screws removed and the center pad plastic support came out for modification. It is basically an upper and lower segment. Upper area holds the center pad in place but the lower area is unused. This photo shows the plastic support with the pad removed and off to the left.

I cut off the lower portion as high as possible to provide room for the speaker box later:

Here is the speaker box with the 8 inch Sony Xplod Subwoofer:

..and resting below the modified centerpad support to show sizing:

And finally with the entire setup mounted behind the AC/Cooler and ready for the seats to go back in. Angle brackets hold it solid to the back steel wall:

An amplifier is needed to drive the speaker. I had hoped to use the amplifier to also drive the 6x9 speakers so I bought a 4 channel amp. The amp is a DHD 200 watt 4-channel unit and was chosen partly because of its relative small flat size. The Alpine head unit in the dash had low level RCA connections to provide signal to the amp so I had to run a pair of shielded wires from the dash to the trunk and used the center tunnel area covers to provide the path. Since I am only using one subwoofer, I had to combine both left and right signals just before the amp in order to get L/R bass sounds from the one subwoofer. Some people say only one channel/side is needed but I disagree and didn't want only bass from one channel. Doing so however meant that I could not use the other channels of the amplifier to drive the 6x9s as they would then be mono, not stereo. I could use high level (speaker level) signals on the secondary portion of the amp to drive the 6x9s independently of the subwoofer. The head unit is already 35 watts and seems plenty for the 6x9s anyway at least for now. So the amp is overkill but it does work well for the subwoofer. The amp was mounted to the back of the trunk on the wall and behind the felt panel. Like I keep saying, I really wanted no space taken and nothing seen. Looking in the trunk or the interior of the car you'd never know anything was added to enhance the sound system.
This is the amplifier mounted to the inside trunk wall (felt panel not yet remounted):

Useful related links:

My BMW 635 page
How to install a performance chip
How to add a central locking switch
How to cure a fuel tank leak
How to fix the sunvisor mirror light
How to make a cup holder for your 635
How to make a front end frame lift
Another BMW I currently own. An 81 320i.
5-speed conversion process for 2002's.
633/635 (E24) Forum on the Web. Oustanding source of info from enthusiasts.
Mesa Performance Parts - BMW/Porsche Parts
G-tech performance meter.

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