I am not certain precisely how much changed between 1964
in regard to the switch, but in my Overlander that switch also controlls the power supply to:
- 12-Volt porch light outlet -- this was to accommodate the accessory home-style porch light that screws into the outlet.
- both roof mounted vent fans
- the original water pump (for easier access, my water pump was relocated next to the water tank so this no longer controlls my water pum).
- the electric hitch jack -- this may be something of a fluke since this feature was added as an aftermarket accessory ten years ago.
My theory has been that the switch was connected to the water pump and ceiling vent fans to be something of a fail-safe to protect against problems that might arise should the power be continually supplied to those devices while underway.
Good luck in sorting out your wiring!