Ford experts - rear turn signal output to trailer stopped

Years ago a friend from Wyoming with a new Ford p/u offered to tow my boat to Lake Havasu for me. I hooked the boat up to my p/u and all lights worked when I took it to meet him for the swap. Hooked up to his p/u and the right turn signal didn't work. I had read somewhere about the control module on the new Fords. I slammed the driver's side rear door and the turn signal worked. This had to be about 2009. And I am not making this up.