I had a 79 F-150 that did the same thing. I did everything to try to get rid of it. I changed wires, checked harnesses, added a ground loop isolator,scraped paint off at the ground point. I also used a distribution block to ground 2 or 3 items and then one main ground from that point. What fixed it??? Your ground wire (on whatever unit or component) must be at least the same size as the power wire. Its not a bad idea to use the next size bigger. You also should not have any breaks in your wire and the termination point needs to have an excellent contact point. Meaning you don't need barrel splices and crimps in the equation. This should solve your problem. It did mine and I've always done it since with NO Problems! Keep in mind that you don't want to use ground loop isolators and other crap of that nature.