Hi,
I'm doing a major install on a 93 300d 2.5. The doors are a lousy location for any speaker (although I installed some 5 1/4 woofers just in case). I had to make a template which is a major pain. Running the speaker wire in the door jam brought some expletives as well. I didn't wire them up to the amp and probably won't. I fabricated an entirely new rear deck. I cut an off-center 12.5" square hole in the rear deck for an Infinity Kappa Perfect 10". It requires only a .6 cu ft box. I wanted to hear the bass, not my spare tire. I had a body shop reinforce it with 1.25" angle iron. It's stronger than it was from MB now. My number one priorty is sound with trunk space, because I actually use it sometimes.

But to get to your dash dilemma. . .I removed the dash speakers and cut the cone and magnet off at the top of the speaker basket. This left me a ring with only the clips and the hook that mounts the speaker. I installed an MB Quart DKD 110 discus line 4" in the ring with HD adhesive. The speaker snapped in perfectly and fits behind the stock grille. The sound they make is beautiful. Nice little crossover network right on the speaker. Tin snips work great. If you'd like a better description, let me know.
I took step by step pics and downloaded them to my PC just before the harddrive went south.