Well having had this happen to me before. lets start with basics.

1) make sure you find it. as in well "i moved stuff around and it stopped" is not the answer. two man job. one watching one controlling power on and off battery. Remove known good power from equation by pulling fuses.
2) Since your asking on this site. It means your original wiring is really old. And brittle. So other than a loose connection my bet is on bad insulation.
3) want to eliminate it from happening again. Rewire all the old. And make sure the new wiring is protected from rub especially on metal. And since its a old vehicle there is mountains of metal edges for rubs compared to todays modern vehicles.


After our third electrical fire on old wiring just from driving it often and the insulation rubbing off or breaking. Complete rewire was done and only electrical issues after that were ones cause by laziness on my part. Todays full wire kits come really nice with color coding and most have what the wire goes to printed all down the wire so it make installation really easy other than finding time.