I think the problem is with my truck but I have no idea how to fix it. When hooked up to my truck, the trailer's brake lights work, blinkers work but the running lights do not work. When I hook the trailer up to my dad's truck, all of the lights work. The trailer connects to my truck via a four prong plug (like the post pinned at the top of this section of posts). The four prong plug plugs into a converter (black rubber piece that takes the plug from 4 to 6) that then plugs into my trucks 6 prong outlet. I've swapped out the plug to see if that was the problem but it still did not work (but my plug works on my dad's truck).

So, how do I find the problem? From looking at a wiring diagram it appears the running lights are there own separate wire. But how do I trace where the problem is on my truck? Thanks in advance.