Others may have clearer advice. If it were me, I would start with a multimeter that can measure the presence of 12 volts. Beginning at the tow vehicle's trailer connector, I would check for voltage. Next check the trailer side of the connector. If current is there, then I'd check halfway to the lamp on the trailer. If current is there, go 3/4 of way to lamp, etc. If no current halfway to trailer lamp, keep bisecting the distance in half, until you know where current is, and is not. You may find a cut wire or a short grounding it out. Aside from the metal frame, I'm not sure where it could short. Good luck.
PS, if no current on tow vehicle connector, of course work backwards, bisecting the distance on tow vehicle.
Plan B. Run new wires to lamps, and attach with zip ties.