Chris,
I bought a Blaupunkt interior subwoofer and placed it underneath my passenger seat. It measures 12"x8"x2". The Bass is adequate, loud and clean enough to fill the bottom end especially in the insulated sealed enviroment of an 80's MB like my W124. Kenwood makes a similar subwoofer. Do not let the size fool you! Most people who complain of the size have never connected one up
Get it from crutchfield or some electronics store that has a good return policy, connect it, if you don't like it, send it back. I am thinking of moving the location of the sub to my first aid kit, it should drop in to the space there without the plastic cleanly, no external amps. Two might even fit
Just a thought