Tail/horn fuse

That is not what the diagram shows, either for 71 or 72, so ??

Easy to confirm.........get to the horn relay connector and probe the violet and see if it's "hot." Remove the fuse and see if it goes cold.

I would be checking over the cig lighter, and examining the rear harness for damage. Don't forget that the license lamp operates off the same tail wire, so tail/ marker/ license lamp.

Also pull the rear seat and examine the harness for damage in that area. Pull the rocker cover and look in there.