[AceX]

I know what's going on now. The solenoids are sending a signal through the chassis and into the speakers. Its a rare issue and one that's hard to track down.

I would start by making sure that as much of the system is as electrically isolated as possible, and start moving through each item to see what is getting the signal. My guess is either the deck, or the amp.

One thing you might try (its a long shot) is a ground-loop isolator. They can be had reasonably cheap, and are simple to hook up. If I had one I would send it to you, but alas, I don't.