Juck,

The tuner and amps are located behind the right side trunk carpet panel (on the w124 sedan - not sure if same location on the cabriolet). Easy to remove the tuner - 10 mm headed bolt goes in the bottom of the bracket. Remove it and the tuner slides out of the bracket.
You might look for a dealer (or independent) that has a test unit that you could plug in to find out which module is at fault. Very quick job to hook another one up once you have the carpet out.
Do a search on this forum for 1432 and you'll get some more reading.