I have an 83 300sd and I've put a subwoofer in it and replaced the rear speakers. I was able to replace the rears with 4x6s rather easily. the system sounds great.

__________________
W126 1983 300SD 286,000 miles and ticking
Baby blue exterior Grey MB tex
Recent work:
Replaced air cleaner mounting brackets and heat shields
Replaced alternator, fan and power steering belts
Replaced positive battery terminal
Replaced negative battery terminal and cord
New Duralast Battery

My car needs work.