I had a U-haul trailer hitch installed on a 2006 Volvo S-60. As soon as you plug in the trailer, the brake lights go out, both on the car and the trailer. You have to unplug the trailer, turn off the car, and then re-start the car to get the brake lights to work again. Plug the trailer in again, and out go the brake lights. Tail lights and turn indicators work fine. Checked and tested the trailer thoroughly and it seems fine. What could it be?