[John Landry]

You might want to check the rear brake light bulbs. Some times the element inside the bulb burns and touches the other element. the brake light bulbes are also the running lights. they have a double element bulb. If that is not the problem you can check the trailer light cord plug. take it apart and inspect the wiring. one of the wires might have came loose and is touching the brake light wire. This also could be a ground problem, if so this is hard to detect. You may have to bring it to a shop. Any truck shop could fix it. I worked on 18 wheelers for 30 years.