sorry i don't have a solution in my knowledge for ya, but just thought i'd let you know that i had the same problem with my system that i used to have in my 83 Audi gt. I just dealt with it and popped the face plate off and put it back on.
decided while writing this that i'd google it.......there's a few things it could be from what i found. Loose connection, or low voltage due to weak battery or alternator. Don't jump to the worst of the battery or alternator. Definitely check all your connections throughout the system before the battery and alternator. You can check the voltage with a meter. Google more to find out what your voltage should be reading. Oh and even a rusted tightly secured connection can cause problems sometimes. so a little sand paper on this could help ya out.
hope it all works out! gotta get that boom boom back