[JRTJH]

Go to post #6, look at the top photo image and take note of the two pin immediately adjacent to the keyway guide. They are labeled "12 V POWER" and "TAIL & RUNNING LIGHTS". Using a 10amp fuse, slightly bend the fuse contacts and insert into those two pins on the trailer umbilical plug (NOT THE TRUCK SIDE, USE THE TRAILER SIDE). With a battery turned on in the trailer and/or with shore power, you should have power to the tail/running lights and they should illuminate. If they do, then the problem is with the truck. If they don't illuminate, then you need to focus your troubleshooting on the trailer side of the wiring. Let us know the results of that troubleshooting step and we can then focus on what comes next.