Hey Andy:

Welcome to the Shopforum. I own a 1990 560sel as well. Had my left amp fried first, and later on the right one went away quickly (less than 3 weeks later). A better aftermarket amp is probably better idea. Then again, if you are looking for minimum hassle / elbow grease, just find an used one cheap somewhere and replace it. The stock amp has a 10 pin connector to it and it's a very easy swap.

To remove the bottom cushion, just take out the 2 bolts on the bottom of either side, and it should slide right out. Don't be afraid to use brute force. MB cars are made to withstand a little abuse here and there. I think the cushion only goes partly out, but you should gain enough clearance to look below the seatbacks and find the 3 bolts that someone else has mentioned. Remove the whole seatback and you should see the factory amps on either side.

The 10 pin connectors that goes into the factory amps are a good source for the oem speaker leads. Just connect your after market head unit's speaker outs into either side's connectors. You should run 2 sets of wires to either connector: 1 set of FR (inlcluding + And - ), 1 set of RR. 1 set of FL, 1 set of RL. All the other power connections for your aftermarket unit can be found from the stock connectors of the original becker unit.

I've got a JVC Mp3 unit connected to the oem speakers. It sounds wonderful and is much more convenient than the older unit. I love how you can play 100+ songs with one single cd and switch the songs around with a nifty little remote. The only loss was the ability to extend the antanna in various heights.

Look for posts from Longston and Scott. They helped me a lot with my project.

Good Luck