No fuse needed. The switch only has a load running through it when it is closed. (The pin is pulled) and that would only happen if the trailer broke away or you were testing the breakaway switch or brakes by manually pulling the pin. I would much rather burn some wiring than have the brakes release on a runaway trailer if the fuse blew for some reason.
Will I need a thermal fuse on the black 12v wire, and what size.......thanks
The newer trailers have a 50A self reseting Breaker. Both batteries are tied together then onto the hot buss. The break-a-way has a self reseting fuse as well. 20 or 30A I believe. If the wire is 12GA it will be a 30A. You can get these from a Marine store.
Michelle TAC MT-0
Looking for a 1962 Flying Cloud