Hello Grubbs and welcome to the board.
It be more helpful if you gave us more info about the vehicle you are in question about. I had a similar problem while towing with my 2001 Suburban and after replacing bubs and checking wiring, I found that there are trailer fuses for the tow package wiring. Left Trailer, and Right Trailer fuses and If I remember right it is in the fuse panel located on the drivers side of the dash when you open the drivers door.
Let me know if this helps.
I found a unusual problem with my 94 Burb when setting it up for our new smaller horse trailer.
After putting all new wiring in the trailer, so I know those were right. When I plugged the trailer into the Burb and tried the parking lights they were not working. But if I started the truck and tried the lights again everything worked just fine.
Weird I know but for some reason the truck had to be running for them to work right.
But check your trailering light fuses to, if the truck came with the trailering package as mine did. It should have the fuses.
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