I know this is probably a much-discussed topic, but did not see any threads which specifically addressed my question and so am posting it here; I own a 1985 S-Class [W-126], which has the famously-pathetic factory Becker setup with 4 speakers [believed to be 3.5" or 4" in the dashboard, and 4X6" in the rear shelf]. I do not need ear-spliting sound, but would like to create a system more comparable to a modern high-end factory set-up [ something along the lines of the harman kardon systems found in newer Mercedes vehicles]. I am obviously going to change the head unit [likely with a Blaupunk or Nakamichi cd receiver], and would like to have mb quart coaxials front and rear, along with an amplifier [probably also mb quart]. I am well aware of the conundrum involving 126's [and Mercedes in general] and trunk-mounted subs, given the fuel tank location and so forth, and would rather not be bothered with this at all. My specific question is whether a set of mb quart 7-8" subs woud fit in the rear [or front] door enclosures, and whether this would deliver the good [if not phenominal] sound quality I am seeking. I would rather not have to have any mods made to the rear parcel shelf either. I will not be doing any of the work myself, but rather having an experienced stereo shop handle the installation. I am really asking these questions from the "Is it practical?" and "Does it make sense?" standpoints. Any help is appreciated. Thanks, and Happy New Year to all!

I have a trunk mounted subs and man, 90% ofthe audio places would not even attempt to run wire in the Mercedes, due to the, as you said, the gas tank. After looking at the way the wire is ran, it's not too hard to put in just less room to work in. (On the passenger side of the tank there is a space to run wire through to the passenger compartment and it's all normal from there except you'd probably run it from the engine compartment. and just don't let the wire get pinched by the adjusting rear seats.) Luckily the owner before me spent top dollar to get the wire ran and so all I did was add the two 12 inch subs which gives me and execlent sound in addition to the decent front door speakers. I got lucky.
In your case, what you suggest is actually practical expecially if you don't want to go through the trouble with the tank and or (In my case with the 560) the adjustable rear seats and stuff. For the back I would go with a good set of 4X6 four ways, or five ways if you got the extra change. This will give you a nice deep sound while giving you quality highs. I don't think that round 7 or 8 inch subs would fit in the rear deck without mods being made. But I would believe that a set of 7-8s will fit in the door. I know for sure a six will. Then don't forget the people that's doing the installation should be able to answer these questions more clearly easily. Just hope they aren't skiddish about installing stuff in a Mercedes.