My next audio experiment is mounting a subwoofer in place of the first aid kit. How many have done this, and how do you recommend building the enclosure? What subwoofers would work well in such an enclosure? Thanks for any recommendations/advice.

I had one on my W124
daul 8" sub woofer where the first aid kit is.
I got it from Reus system, and is not expensive either.
best regards

Instead of removing my medical kit and dealing with that aggravation, I bought a small powered subwoofer from Subaru and placed it under passenger seat (will just barely fit)of my 300CD. I understand Blaupunkt also sells a small one. These come nicely packaged.

Not the same bass response but not too shabby. About $250.00

Nic in SC

I put a Kicker Freeair 10" Subwoofer where the medical kit goes on the back deck of my 1977 300D. I used a Freeair Subwoofer because the fuel tank is right underneath the medical kit and there was no way to build an enclosure in the small space. A free air type sub-woofer does not need an enclosure to work, but you do need an air-tight seal between the trunk and driving compartment to get the desired results (or as close to air-tight as you can). If your fuel tank is not right below the medical kit, then you can probably make an enclosure or buy one. An 8" subwoofer speaker needs a enclosure/box measuring about 12" x 12" x 12" (this is a small box, other 8" subwoofers need larger boxes, and larger subwoofers need larger boxes). Crutchfield has lots of trechnical info about subwoofer's for cars and you should read about them before jumping into this project (www.crutchfield.com). Crutchfield also sells subwoofers, amplifiers, subwoofer boxes, etc.
Also, because of my subwoofer's size I had to make spacers to raise the height of the subwoofer so the magnet would not touch the fuel tank. I used 3/4" plywood to make the spacers and used the medical kit's cut out as the template to make spacers. I used silicone to seal between the spacers and also the bottom spacer to the medical kit enclosure. However, your car may not need a spacer(s). I then used a piece of thin, porous cloth (same color as the back deck) to pull over the subwoofer ((I also mounted 6x9 speakers (on plywood spacers) on the back deck)) and tucked the cloth under the back deck (all the way around using thumb tacks to hold the cloth in place) and it came out really nice - looks like a factory installation, and the sound is awesome! The cloth protects the speakers from the damaging S. Texas sun and from prying eyes that may be looking for car stereos to steal.
Another option is to put the subwoofer/enclosure in the trunk and run a sound tube from the subwoofer enclosure/box to the driving compartment (this is done a lot especially when the driving compartment does not have the room to mount subwoofer(s) or someone wants one or several large diameter subwoofers). Crutchfield has info about this type of subwoofer enclosure and mounting.
The suggestion to use a small, self-contained subwoofer is also an option worth exploring. Bazooka Bass Tubes is the name of a company that makes self-contained subwoofers in tubes that can be mounted in a number of positions within the driving compartment (Kenwood, Jensen, Rockford Fosgate, and MTX are other manufactuers making self contained subwoofers). Bazooka Bass Tubes come in a variety of diameters (6", 8", 10"), are fairly compact in size (especially the 6"), and are either powered with a built-in amp or need a separate amplifier to power them.
As far as bass response (or the thump) the best is a subwoofer in its own box that is the correct size (see Crutchfield's explanation about subwoofer enclosures and optimum size based on the manufacturer's guidelines for a particular subwoofer). This type of subwoofer can be mounted in the trunk (with a sound tube run into the driving compartment) or you may be able to build an adequate enclosure below the medical kit. The next best is the freeair type subwoofer and does not need an enclosure (my choice because I did not want the subwoofer enclosure/box in my trunk). The Bazooka Bass Tube probably has the least bass response of the bunch, but it is the easiest to mount and may meet your needs for bass.
Subwoofer sound waves are non-directional, they impart the "feel" or "thump" of the music - so the mounting position is not nearly as critical as it is with other types of speakers.
You may want to put bass blockers on your other speakers since the subwoofer will handle those frequencies. Bass blockers will usually make your other speakers sound better because they will not have to reproduce subwoofer frequencies that they were not made to do (look at the frequency response curve for your speakers - if you still have the box the speakers came in - if you do not have a frequency response curve for your speakers, then take my word for it that the drop-off at about 100 Hz is pretty much straight down for 6x9 speakers and smaller speakers drop-off at higher than 100 Hz frequencies - like 150-200 Hz). Also, bass frequencies need a lot of power and using bass blockers will allow your amp to work less to make the same volume level (this fact is why most subwoofer amps have so much more power than amps for the other speakers - my subwoofer amp is rated at 100 watts RMS/170 peak and I drive my other four speakers with a 4x35 watt amp).
With a separate amp for your subwoofer you also need a crossover to separate subwoofer frequencies from the other frequencies. You may have an amp with a built in crossover or you may have a separate crossover. I used 125 Hz as my crossover point with good results (purists will insist that the crossover point be lower - like 60 Hz - but we're talking about cars that are not perfect listening environments). The bass blockers for my 6x9 speakers are 100 Hz, so I have a good transition (or lack of loss of sound) using 125 Hz as my crossover point. My four inch speakers have 200 Hz bass blockers.
There is a lot more I could write about subwoofers in cars, but this should help. The Crutchfield site has lots of useful info and is a good place to start.
Good luck! !

I am considering putting in one Kicker F8C4 (Free air, 8 inches, 4 ohms) in a 1997 E to add a little bass and connecting about 150 watts to it.
Do you think that would give good results? Or should I use at least two 8" subs? For that matter, should I consider a 10" free air (although I do not want to cut in the deck) Thanks.