I have seen more than one truck with broken pins inside the connector located just under the left rear taillight. this is where the rear (tail, stop and reverse) lights wire harness connects to the trailer hitch connector (whichever one you have 4 or 7 pin). If you take the connector apart and see corrosion or broken pins, best bet is to cut the connector out and hardwire everthing, only 5 wires
Don't mistake it for the connecotr to the main harness which is about 1 foot farthur forward and on the inside of the frame rail (you have to lower the spare to see it). this is the wrong one.