Originally Posted by Tom Williams
If the red wire was 12 volts at one time why is it dead now?
Possibilities: loose wire, broken wire, disconnected wire, blown fuse, loose fuse, corrosion on wires or fuse, bad terminal on converter or other 12 v. source, tested improperly (may not have found a good ground on tongue).
Test each end of wire once you find the other end. It may well be simpler to just attach a new, fused wire to the batteries. The fuses for the newer jacks are pretty high amperage and are hard to find except at RV stores. I can't recall how many amps, unfortunately.