I put backup lights on my car in a pre-Invasion scramble, so I don't have written notes on it. I do remember that the aft end of the wire is in the trunk, nestled up next to the crossover tube, a little left of center. Standard bullet connector. The other end I believe is in the engine bay, near the fuse block or near at the firewall near the master cylinders... one of those two places. I think I picked up power from the fuse block, went down and back to the trans switch, and attaching to the firewall-area bullet connector. At the back, I ended up using through-drilled bolts instead of making another hole for wires. This was a pain in the butt. Note that I was not installing authentic, or even repro, Wipac's, and was not installing them on, shall we say, a car of scrupulous pedigree. I just wanted to see behind me.
I think the wire was purple/green, but that's just a guess. And is this the circuit that's not in the wiring diagrams? Or is it the OD circuit...
Anyways... I would recommend thorough testing of the switch operation via gearshift before installing the trans. I also recommend installing the switch wiring harness before trans installation.
Also check the wattage of your lamps, and calculate the combined current load on the wire. It would be easy to smoke it with the wrong bulbs.