My prediction, based on my recent tail light headaches: the left and right are not wired the same "up and down." I had illogically working tail lights (with dual filaments) and learned that I had one of the sockets turned 180° relative to the other one. This fouled up the brake/tail light operation so that I didn't have logical brights and dims.
Using a 12v power source, I learned which contact connected to which filament. Using a test light I learned which prong was for the bright and which was for the dim. One twisted socket later and I was back in business.
Bill
