Cainja seems you and I are simultaneously working on the same problems with our 83 300D's. I've had mine for a month and like you have been fixing the interior lights and cruise control. I'm no expert here but this is worth a shot. If the problem is the amplifier resoldering the printed wiring board or replacing 3 parts worth $3 can probably fix it. I read this on another thread on this site. The parts are available at radio shack and you can do the resoldering.

My cruise control has a different set of problems. It works but it's so herky jerky it could make a sailor seasick. I think it's too much slop in the linkage so I'm going to replace the ball clips next.