Ok, popped the speaker out of the dash. Man, this is a wacky design. I could see putting a 4x6, but modifying the 4" length to make it the full 4" instead of the 3.75 it is now. Hmmmmm. Looks like about 1.75" deep.

These Kenwoods might work.... Don't know how great they would sound. KFC-4625C

http://www.crutchfield.com/cgi-bin/S-puh9MSzP1VR/ProdView.asp?s=0&c=3&g=51500&I=113KFC4625&o=&a=

JBL's got one too, although slightly deeper. GTO6420

http://www.crutchfield.com/cgi-bin/S-puh9MSzP1VR/ProdView.asp?s=0&c=3&g=51400&I=109GTO6420&o=&a=

Hmm, a few other's on Crutchfield, too. Hmmmmmmmmmmmmmmmm.

Got a message back from MBCoupes, the 1988 and newer 560 SEL's have speakers in the front doors--just transplant the inside part of the door (or buy a newer car! ...)

You could probably find a 4x6 plate that you could mount a 3.5" in, but yeah, it might not be worth the work.

Without doing the doors, what else do you have?