Amazing what a photo hides, I would have never picked that up on your 80 Nigel. Perhaps your old truck would be good around the speedway track.
It's been that long I can't recall which way they go, so you might have to go in reverse. A bit like these electrical systems. There was no real reason to go back, just they way it was intended I guess. I had no idea, but was the first thing pointed out when we were fixing the horn. It seems in my case it was a matter of just spinning the battery around and figuring out the junction box from the wiring diagram in the manual. I think due to the wires almost unidentifiable by their colours the uncle may have just did what worked. It took a little figuring with the volt meter but all done in a hour or two. By the way the horn improved by cleaning the contact in the steering wheel, but did seem to also get better with positive earth.

Going on Lucky's history, I'd strongly recommend keep the 80 mobile when bush fires are around.