Guys, I bought an enclosed car trailer a couple of years ago and the interior lights never worked. The previous owner said they didn’t work for him and gave me wiring schematics. I didn’t spend alot of time on it but last week I took the trailer for a professional service on axles and brakes; they fixed the problem in about 3 minutes. The fellow raised the hood ON MY TRUCK and removed the fusebox cover on the driver’s side. He then reached into the wire loom nearby and pulled out a length of red wire that had a connector on the end. That wire went on the middle post of the fuse box. He put a fuse in the circuit and VOILA... lights in the trailer. I somehow overlooked that if it was in the owner’s manual. My truck is a 2008 2500 HD w/ the towing package. I hope this helps someone.