do you know which side is popping fuse ? doesn`happen to be right side does it? I think i would be tempted to disconnect all trailer wiring now .also check the light housings themselves .take the light housings right out of vehicle to check wiring leading up to them. pinching or rubbing just look for wires that may be pinched or rubbed and thus allowing bare wire to come in contact with metal surface and popping fuse. Have you replaced the signal light flasher?