There are wiring standards for each type of trailer connector. Sometimes people don't research the wiring standard and create their own.
On my 2003 F150 with the factory towing package and factory trailer wiring the trailer lights are fused independently from the truck lights. That should be illustrated in your owner’s manual. On my truck the trailer fuses are under the hood in the power distribution box. There are also trailer towing relays for trailer back-up lights, electric brakes, and 12v trailer battery charge. All that's in the owner’s manual, good luck!
If you don’t have an owner’s manual you can check each fuse with a 12 volt test light.