trailer running lights not working at plug on truck

"Daleo Kelso this will only work if your truck has the trailer towing package. If it has been wired aftermarket, you'll have to dig a bit for it. For the truck equipped with the package:

Fuse F31 will be for the parking lights on the trailer connector and Fuse F58 is for the trailer connector backup lights."

Check #67 under the hood behind radiator. There is a fuse box there. I had same problem. It's a 20 amp blue 1cmx1cm square style with clear window at top. A simple continuity test will tell if it's bad.