I would just buy a 7 pin trailer and wire an 1157 bulb across the brake circuit.
That's pin 3 they are usually numbered.

Perhaps one of those receiver mounted brake lights (non-LED).

 

I don't know the pin numbers off the top of my head, but as you look into the socket of the truck, brake supply is at the 5 o'clock position and ground is immediately left at 7 o'clock. The center pin is reverse lights/aux.