From what I can tell, my '84 300sd has a bad amp (accelerates but doesnt hold speed). I know where its located under the drivers side dash, on the little bracket. But what is the other module on the same bracket (also in a metal case, but smaller)? I want to find out exactly what I'm looking at before I start unplugging and removing things.

I haven't had the SD apart, but the SDL has the seat belt warning buzzer attached to the bracket that holds the amplifier. You can remove the bracket, with the amplifier and the buzzer. Leaving the buzzer disconnected won't bother anything AFAIK.

Once located, its easy to unplug, and pry the case open and slide the PCB down/out so that it can be resoldered. No need to unbolt the case.
This will fix most cruise amps. The ICs are marked and its not too expensive to "shotgun" the whole board.

I may have a good spare if needed.

Good luck.