sounds very much something added on by the PO after he had a problem that resulted in draining the battery your only alternative [beside rewiring the truck as original] is to take that switch to a good parts store and see if they can match it
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics
your only alternative [beside rewiring the truck as original] is to take that switch to a good parts store and see if they can match it