Follow the trailer plug to the source… i.e. where was it added to the wiring harness. If they used these horrible, nasty, useless things - scotch locs - assume THAT is the problem;
Scotch Locs, in theory, cut thru the insulation and make the connection. Sometimes they cut thru the wire as well but they ALWAYS let moisture in so the wires corrode. The wires themselves may become the fuses and they burned up. OR the ground wire burned up. In any event, you need to follow the wires to their connection points.