I learned many years ago that when in doubt, check the grounds. I had a 24 ft house trailer that was driving me nuts. I had checked every electrical connection, fuse, whatever. The electrical gremlins would not be controlled. Finally I disconnected the main ground and found that the steel frame was rusty making a poor ground connection. I cleaned it up, reconnected the ground, and all of my electrical problems disappeared. I now always start by checking the ground connections if having any sort or electrical problem.
Side note; I also ran a ground wire from the Le Mans gasoline filler cap to the frame on the Cobra just to make sure that the filler is properly grounded. It may not be necessary but it only took a couple of feet of wire, a couple of crimp connectors, and 10 minutes of work. Better safe than sorry.
Wayne