Many thanks for the all the suggestions I received from you all. I just got the owners manual by mail from MBenz this pm and immediately referred to section for seat cushion removal (p 99). It said "On vehicles with power rear seats, the rear seat cushion can best be removed and installed by an authorized MERCEDES BENZ dealer"! Bummer
This probably explains why the previous owner never had this speaker/amps problem repaired.
I'll post the results and my experience if and when I decide to go to the dealer...
Andy