Hello and welcome to JustAnswer!
What kind of problems are you having with your trailer lights?
Ok great. The first thing you'll want to do is disconnect the battery and then check and see if the turn signal works.
The lighitng on your truck is controlled by the TIPM, the fuse box/module under the hood. When it sees a high current draw in one of the circuits it will disable that circuit and set a trouble code. When it does this it will disable that circuit until the code is cleared.
If the battery disconnect fixes the problem you'll want to see a dealer. The software in the TIPM was revised to be more tolerant of this problem and not disable the circuits so easily.
If a battery disconnect doesn't bring the lights back you either have a wire between the TIPM and the trailer connector that is broken, or the driver circuit in the TIPM is burnt out. In that case we can do some electrical testing with a test light if you're up for it.
Ok. The next place we should go is a junction connector just forward of the driver's door. Slide underneath the truck and look at the area where the floor pan starts to curve up to the firewall, outboard of the left frame rail. You will see a large junction connector there. This area will sometimes show some damage from road debris. The wires can be cut, and they can also be corroded inside. All it takes is a pinhole from a rock in the wire and the moisture will eat it from the inside out. Grab your test light, check for power on the front side of the connector on the yellow wire in pin 2 of the connector. It will be next to the green wire which is the right turn signal. If you have power on the front side, check for power on the back. This will ensure we have power through the connector, no damage inside. If there is power this far, you've got a problem from there to the back. In that case you could look for the break, or run a new wire.
If you don't have power at that connector, the next thing we'll have to do is pull the fuse box up and test for power right there. Let me know how it goes and we'll pull the fuse box up if we need to.
Ok. Pull the fuse box up. Find pin 18 in connector 3, the same yellow wire. if you have power there, the wire is broken between the box and that connector down there. If you have no power, the TIPM is bad.
Here is the diagram for the connnector locations again.
Here is a pinout of connector three to help you find the wire.