41 Pickup switch debate

[411Ton]

appreciate any insights as to this switch. Debate raging on a Facebook chat about the true nature/function, was it original or not, etc. Does anyone know if it's for "dash lights" as many argue, or heater, as others argue, it is original or not, and lastly, wiring diagram to help the original poster fix. I have one in my 41 3/4 ton also, but it has no wires going to it so I have no idea what it's for either.