dave:
How do you identify the spare wires? I found a wire labelled 12v, and obviously a spare, in the basement, just ahead of the rear bulkhead. I used that one to get a 12v outlet in the basement. For the brake controller, I had the seller install it, but his tech just ran a new wire all the way, threaded thru stuff underneath, and in places not in sensible places, so I have had to move parts of it. I would far rather redo it to a spare, if only I could find one.