Re: trailer lights
Yup, bad ground. The tail light and brake/turn signal light negatives are connected together via the frame. As long as the frame is well grounded, the circuits function independently. If the frame ground is poor, the tail lights will backfeed through the brake/turn signal filaments in the trailer and then through the filaments in the tow vehicle to get to the tow vehicle ground. Because the filaments in the tail lights are not as bright (have a higher resistance) the current that they carry is not enough to light the brake/turn filaments, so it goes unnoticed. When you step on the brakes or apply the turn signal, the tail lights lose their backdoor ground path and they go off. When things get weird, it's usually the ground that's causing the problem. - Grandad
