Last week my car was in the dealers having a piece of door trim replaced under warranty, so I took the opportunity to apply some adhesive speaker cabinet foam to the outer edge of the door cards before they were put back onto the car.
After reading up about speaker enclosures on the internet, I figured that the lack of bass was due to the fact that the car doors just aren't sealed well enough to offer decent bass. Basically, all the air pressure generated by the speaker that is needed for good strong bass notes was leaking out from the edges of the door.
The material is similar to the draft excluder foam that you can buy for home use, but it's thinner (approx 2mm thick) and is approx 2cm wide. It has an adhesive backing.
I simply ran the foam around the outer edge of the inside of the door card, butting it up neatly against the plastic, so that it's not visible from the outside when the door is put back together. This prevents bass leakage from the inside of the door, and makes the door behave more like a speaker cabinet.
I also made a flange from the foam around the speaker cover part of the door, by running a loop of foam around the inside of the speaker cover, and on the outside too, then pressed both adhesive faces together to prevent the foam from sticking to the speaker. This ensures that the sound emitted from the front of the speaker exits only through the front of the door, and doesn't leak into the door itself.
For an investment of just a few quid, and 10 minutes work, I really think that this is an essential part of the speaker upgrade, alongside insulating the doors. The bass is now solid and sorted, and stereo seperation is spot-on.
As usual, I didn't take any photo's though! Sorry!
I got the foam from CPC (online & mail order), 1 roll is more than enough for 2 doors. When I get chance, I'll post the part number.