smcser

The white wire is usually the ground wire. Run a ground wire from the plug directly to the wheel magnets. And a ground from the plug to the frame. Do this also on the truck!
Also check the brake shoes and adjustment. No rust on the drums for the magnets to contact to. It will be very hard to keep the drums from rusting but try to flush as much as possible and never store wet.

sleev-les

Sounds like a ground. I'd write more of a response, but would be repeating what others have already said.