E30 Radio / Amp problem

I have a problem that has drawn out over several months. The symptom is the radio has proper channel indication ie the display shows the numbers and when selected to a good station a "ST" indication as well, meaning the radio is receiving a signal. I have no audio output or static. Cut to the chase....a RAP on the audio amp fixes the problem until I shut the ignition off. I picked up a used amplifier & installed it today and my problem remains.

Any thoughts?