Has this worked in the past for you, or are you just getting your truck and trailer set up to go for the first time? You might start with testing at the plug on your truck with a test light or volt meter. You should have voltage to an auxilary terminal. Depending on how your truck is wired it can be hot all the time, or just when the ignition is on. On my Ford, the aux power is fused, and the fuse is in the fuse box under the hood. That would be the first place I would check. Then I guess next would be pull the plug out and see if you even have a wire in to the aux. terminal, and if so to see which terminal it is wired into. You'll want tomake sure your truck and trailer are both the same.