We had a tail light on the back of our 5th wheel go out. We replaced it and went on our way. Next morning we hitch up and no lights on trailer at all. No running lights or brake lights etc. Voltage meter at the truck connector shows no power. Took truck to repair shop, fuses all good. He says we need new brake wiring harness. Ford is closed as is weekend. Napa closed also. So we wait till Monday to go to Ford for part. We have disconnected trailer so we can just take truck in. Any thoughts on this? Seems weird.
Being that all the lights are dead though, I would wonder about maybe a bad ground wire.
You can use a fuse and turn on the lamps, like for example here is how I turn on the marker lights - https://youtu.be/O0dzsnPGB08?t=825 or can use OHM meter to test for continuity.
[Update, turned out we needed a whole new wiring harness. Replaced, all is well. Was not inexpensive though. Had to go to Ford dealership as no one else where we were had the part. Ah well.