Trailer Brake or Truck Problem?

thomasmnile wrote:
Thanks 'bird. This is gonna be a daunting task, there's a lot of wiring to check. Not related to the brake issue, but our last trip out the 10 amp fuse on the truck that protects the circuit feeding the clearance lamps on the trailer blew. Replaced that and no problem since. This truck has developed a history of electrical gremlins, this brake issue is just the latest. May well be the trailer brakes but the truck's recent history isn't helping.........

Dollars to donuts you have chafed wires on the trailer that are causing intermittent short circuits.
Check the ENTIRE wiring, but focus mainly anywhere the wires pass through a hole in the frame or into the axles.
For the brake issue, this has nothing to do with the truck electrics, the P3 is a standalone device. If it is getting power, a signal from the brake light switch and the brake wire to the 7-pin is good, then the problem HAS to be the trailer.

As for your comment " truck brakes have plenty of whoa for truck and trailer " that may be OK for routine slowing and stopping but NOT for an emergency stop.
If your trailer weighs the same as the truck, your stopping distance is doubled, if the trailer is more then the stopping distance is even longer.
Get it fixed, the risk is not worth it!