A light in series is an excellent short detector. Have someone watch the light while you inspect and wiggle the wiring. If the light goes out you've found it. A likely place for shorts is loose wires in the connector plug, pull the covers and recheck and leave it open.
Isolate the problem to the lights by disconnecting the electric brake wires in the now open plug. Do you have a battery for those brakes and if so what condition is it in?
If all else fails cut the light wire near the rear and continue forward to find the problem. If that fails run temporary wiring on any convient route like over the roof and resolve later. I've done that because it was the quickest way to continue the trip.
