Jerry,
I'd bet it's a 6v fan. Maybe someone has a Blue Chip GMC accessory/option brochure? GMC changed all models to 12v negative ground by 1956.
The earlier 6v systems would have been positive ground.
A 6v fan would probably run on 12v negative ground - my guess is that it would be a faster RPM.
Maybe the Moderator will move this to the electrical forum?
