Prostock:
Electric trailer brakes normally draw 3 amps per brake. If your trailer has tandem axles and brakes on each axle (not all do) the trailer brakes should draw up to but not exceed 12 amps (18 for a triple axle) when fully applied. Insert a multimeter/ampmeter in series with the trailer brake wire and have someone manually apply the trailer brakes while you read the meter. If the brakes draw in excess of 12 amps you may have a frayed wire shorted to ground somewhere. If the current draw is less than 12 amps but the brake controller signals an overload condition, the brake controller is either faulty or underrated for the application.
Harvey