You could have a chafed pair of wires that wore through their insulation at some point. I've experienced that in old English sports cars.
I think I have a similar problem inside the walls on one circuit in my trailer, and I dread the idea of taking the walls apart to check the wire run for a short-circuit. That circuit probably has 30 ft. of wire in it, and three 12V
lights (both reading lights over the Lucy and Desi beds, and the bathroom overhead light).
With luck, I hope you (and maybe me too) may find the short right at the end of the wire, either at the switch end or at the light fixture end. My luck is rarely that good with old cars or old trailers.
Good luck and good hunting,