OK, here's the aftermath. The factory holes were enlarged for some a/d/s speakers installed by the previous owner. So, the Rainbows wouldn't fit properly. I tried fitting some 5.25 Quarts I had but the basket is too big, it hits the firewall. So I decided to use the Infinity 4-inch coaxials I picked up fairly cheap ($65 new, IIRC), model 42.5i.
Of course they were much too small for the gaping hole in the dash. So I made a baffle plate from sheet metal. You have to do this or any little bit of bass the 4's produce would be lost, since speakers need a baffle to separate the front sound wave from the rear of the speaker cone. Making the baffle was EXTREMELY tedious... had to make a paper pattern first, then transfer that to sheet metal, and painstakingly cut, fit, trim, adjust, etc until they fit properly. I think it took at least 3 hours per side (seriously). I'll post some photos below of before, during, and after. My amp isn't connected yet so I can't comment on sound quality, I did test them with a home stereo and they do work.
My other car, with an unmolested dash, will be getting Rainbows. Sure, they cost TWICE what most other decent 4's cost, but the savings on install make it worth it to me (6+ hours of my time is worth at LEAST the ~$75 difference.) I'd love to go with the full Jehnert pro setup, but at $1000 shipped from Europe that's too steep for me. They're looking for a US distributor which would hopefully allow for a lower price over here (hint hint, Scott!!)
Photos of the install, plus the 143A alternator upgrade, are at this URL (more photos will be added in late December and January as I finish more of the install):
http://www.meimann.com/images/mercedes/W124_stereo/
Best regards,