Do the back up lights work? Also, if you disconnect the trailer from the truck, does it make a difference?
Since the third brake light is working, we know it isn't the switch. Do me a favor. Check the fuse I circled in pic 1.
If the fuse is good, check for power to the dark green wire in pic 2 and confirm continuity to ground as well. In pic 3, check for power to the yellow wire and for continuity to ground.
Let me know what you find. Also, let me know if any of the other tail lights are working.
Images (Click to enlarge)
Sunday, June 28th, 2020 AT 11:00 PM