When ever you are faced with an open DC circuit the best thing to do is start at one end and work you way through the circuit teasting as you go.
In this case I would start at the brakes. With a long wire attached to the trailer Positive 12 volts. Open the Feed Lead at each of the backer plate and apply 12 volt
to it. This will test the brakes themselves. Once you are sure the brakes themselves work I would go forward and open the inspection port.m Again apply 12 volts to the brake location point on the pig tail. With a test lamp or volt meter check for 12 volts in the inspection port. If found remove the 12 volts from the pig tail just to make sure you have the right wire. If the brakes and the pig tail check out OK it is most likely a broken wire in the trailer. Rather than try to replace the any broken wire inside the trailer I would run New Wires back on the underside of the trailer.
When I run wires on the outside of the trailer I use 1/4 in soft tubing attached to the underside and fish the wire through the tubing.
A tip on fishing a new wire. Tie a knot in the end of a string long enough to go throught the tubing. Insert the knot in the tubing and with compressed air blow the string through the tubing. It will go and it will go fast so hold the other end or you will blow it all the way through. Use the string to pull the new wire through.