Dead short in the trailer wiring or one of the trailer light sockets.
Check the brake light fuse. This year Ford trucks have a safety mechanism that prevents shifting out of park without pushing the brake pedal. If a brake lamp or trailer wiring is damaged can it blow the fuse and it won't allow a shift from park.
A short in the brake light circuit.
A blown fuse is always a sign of excessive current flow. In this case, it's probably caused by a short circuit in the brake light wiring.
A blow 15 amp fuse in the under dash fuse panel causes that i think it is fuse # 24, goes to csmt or something like that
Tail light assembly short
It turned out the coupling for the trailer lights was all corroded. It didn't work to just clean it, we had to entirely disconnect it. Works fine now.
Because lighting up the brake lights of both the car and the trailer pulls too much power. If the problem is a specific trailer then your trailer may be mis-wired. You may also have a mis-wired trailer wire adapter.
A short, water getting in the light socket. exc.
short circuits in the turn signal and brake light circuits
short in brake light switch located on brake pedal
Check to see if water is getting into the brake light sockets causing them to short to ground