I just replaced a set of 3-way speakers in a Mercedes E-Class (W210) and C-Class (202). I was so happy to find drop in replacements, hand made by a German company called Rainbow Car Audio. The sound quality is top of the line and I have installed DLS and Focal in the past and these are by far the best sounding. After looking at their web page I found that they have drop in speaker sets for just about every German auto manufactured, with no retrofitting needed. I guess they have designed each set specifically for each model of car and tuned each crossover accordingly.
Thout this might be of interest to some of you with similar problems as my clients who do not want to retrofit their cars...

-Mark
Metric Wrench Autos

P.S. I found most of the info in Google. Their web page is www.rainbow-audio.de and they have their dealers listed in the USA.