I don't know what you currently have in your car or where you eventually want to go with it, but what I did was I ditched the oem front and rear shelf speakers as they were rubbish. I also ditched the Audio10 as that is also a fairly useless unit, even for its time.
I replaced the rears with Clarions, the fronts with JBLs and the tweeters with Pioneers, the head end is an Alpine, and also an amp in the boot, and it doesn't sound too bad. Certainly a massive improvement to what was there as stock and didn't cost too much either.
The bass response fron the rear is a bit poor but the W202 does not lend itself well to fitting bigger speakers in the rear shelf, unless you start to do some metalwork, and I really did not want to get into carving up the rear shelf, but, could be done.
Sticking bass boxes in the boot are also a bit of a waste of time and money. In the W202 there is soo much insulation etc between the boot and the cabin that the sound simply does not penetrate through.
The Germans in this era may have known how to build a decent car but didn't appear to have a clue as to how to fit in a half decent audio system. This car came out of the factory packed with an eight speaker system that basically was of very poor quality hence poor sound.