Check if the wire harness to the Brake and marker lights are not damaged by sharp corners ( like the opening the taillight assy screws into) , this could happen when changing burnt lights. Sometimes a bare wire takes a bit to find a place to ground out. Corrosion in light sockets or trailer sockets can ground power to the wrong place as well. The trailer socket harness hangs low enough to scrape off road hazards and is easy to see or check out. If the fuse pops the second you use the turn signals or hazards it should be pretty easy to spot the short. Take a look and tell us what you find.