OK got my new electronic troubleshooting toy in the mail yesterday (tone generator and probe). I traced the 3rd wire and it is connected to the tow vehicle plug harness (7pin). A quick continuity check with my trusty D'Arsonval meter confirmed it.
I did have in line fuses installed in the wire coming from the converter. They looked cheesy. Newer style automotive push in fuses. I'm replacing both one at front exposed to elements, the other in the converter compartment.
Yes the little break away brake box has a wire coming from it. I haven't traced it yet. I plan on replacing the broken cable too. For now the trailer brakes are working.
Thats great idea to check for leaks with positive air pressure! You guys n gals are full of great ideas!! Keep em coming!
Thank you
RK