Start with a fuse check. Then go to the connection box on the truck and check it for power out of the leads. If they work then it's in the trailer. Let us know if you need more info. I will return with an entry from my black book of info.
If all the lights are working OK on your truck... I would start from your trailer harness plug in on the truck.
Use a test light or malty meter and test your contacts at the plug at the trks back bumper. (I'm sure they're fine) Most cases, it's usually the wiring on the trailer that has the problem.
Keep us updated on your progress.
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