Pathogen, this is one of the very typical problem for these cars. Mine is a 1991 560 SEL and I replace the amp myself. It is fairly easy if you can turn acoupel screws loose and have patients.

MB uses a separated amp for each channel. They are behid the seat back of the back seat. What you do is to switch the amp on the good side to the failed side to confirm it is the amp. I have typed a detailed procedure so many times so you should be able to do a search to find the write up.

If you go with Becker, I think they charge you $125 plus core plus shipping for a rebuilt one. I got two from Ebay. If you really could not find one, I am willing to sell this spare to you.