I recently purchased a MB SL380 for my wife for her birthday just recently. The previous owner "upgraded" the stock radio but putting in a basic cd player about 10 years ago and decided not to replace the 4" speakers in the dash but rather install some 6X9 in the space behind the rear seats. The plan is to install a new retro sound headunit with bluetooth capabilities, replace the 4" speakers with some upgraded stock replacements, and install a SSA four channel amplifier to power the speakers. In the near future I would like to install a small subwoofer in the trunk for some lows, the 4" speakers are going to provide anything below 100hz.
Pictures posted below in the next post, here is a link to the new headunit: