A lot of wiring on these truck don't make sence. For example if the battery light in the dash goes bad the alt. stops charging.

Hope that was your issue

 

Look at the wiring diagram and it'll make sense. As I said, same issue I had except it was a trailer connector that caused the problem. The power from the fuse goes to the TPS and also controls the reverse lights. The wiring on these rigs can make your head spin.