I would start checking for corrosion somewhere. Check the backside of the truck plug for green stuff. Unplug the harness from the socket and check for power there first.
There is a connector up behind the LF fender splash shield. Find the one that feeds the trailer hook up and unplug it. Check for right side turn power there to figure out if your issue is in the power center or in the harness that goes the length of the truck. Do not prick any wires with a test light. You can do this by un-plugging stuff.
Take your time and figure out how to release the connectors without breaking any of the locking tabs.
Mike.