I had my 2018 Lariat in for a clunk in the rear end, and to have the factory trailer brake controller installed. While at the dealer, they gave me a 2019 XLT for a loaner. I hooked up my trailer to the XLT, pressed the brake for a few seconds, and the truck recognized the trailer. The lift light (not turn signals or brake) came on, and I had power to the trailer (12V jack, interior lights,etc.).

I picked up my truck yesterday after they replaced the shocks (rear end clunk) and installed the TBC. I hooked it to the trailer, followed the same procedure as the loaner truck, and there is no 12V to the trailer (jack, interior lights, etc.). Took it back to dealer this morning and they found that at idle in Park, there is no 12V to the plug, but in any other gear (or I believe elevated RPM in Park), there is 12V. So to use the 12V at the trailer, I would need to have the truck in Neutral with the parking brake on, or have someone else in the truck holding the brake down. They are looking for solutions now...any ideas?

 

There is the shade tree mechanic's method. Run a fused 10g wire from the battery back there. Not elegant, but it works. You can even run it through a 12v relay that is hooked to a switched circuit so the 12v wire is not hot unless the switch is turned on. That would keep a trailer from draining your truck battery if left hooked up overnight.

That being said, I would give the dealer a little time to figure it out before doing something like this.