This is a guess, but I think it's likely that the amplifier is damaged. You may want to search on terms like "W203 Bose amp" to see whether there is a procedure for testing the amp. It should be fairly easy to access. If the coupes are similar to the sedans, you need to remove the equivalent of the trunk liner.

The amps are available used, but you would be replacing one 23-24-year-old part with another old part. It may be better to replace it with a good aftermarket amplifier, but that may require some rewiring. An audio retailer like Crutchfield may have some suggestions.

Good luck.

 

It could also be a break in the fiber optic loop. If you unplug any fiber connector, you should see light on one of them. If not, then there is either a bad connection, broken "wire" or a bad device in the loop.