Ok i have been looking at the diagrams, I think the problem must be it your trailer tow connector on the back of the connector ( if it hasn't been replaced with another style) the harness plugs into the connector. I have had the clip break and the connector back off.
Here are the diagrams of the system.
I wasn't asking about the truck , I was asking about the trailer lights does anything work?
Brake lights, turn signals. marker lights and reverse lights are all out