Hello,
I had a 92 S class, I loved that car. You got a good car! one of the last ones.
My advice on the speaker in your rear door would more than likely be that the wiring could be a problem. I am sure you may have already checked that. The cars come with a bose unit, there is a large amp located above the drivers side rear wheel well in the trunk, it is a black box with heat sinks on it. When these fail they usually cause speakers to go out. My car had a very large Sony amp put in it's place since the Bose unit was not available. The unit is mounted behind the gray trunk material, it is in front of your vacuum pump.
Removing your door panel is pretty simple, just remember that you need to disconnect your battery first. Then remove the screws near the latch on the door, then the cover for the interior door handle, and there should be some cap with screws under them near the armrest, It just pops off. Be careful not to pry too hard, be patient and go slow. After all the screws are out you can then remove the panel by popping the snaps around the outside of the panel trim, after these are popped up you can raise it up and over where it goes down into the window channel. My S was a lot older than yours and those Bose speakers were all in great shape, so I would check the wiring and the amp first before removing the panel. Good luck!
Brian