my 46 GMC was the same as a Chev, as Gene says, the one behind the dash louver is to light the dash controls, and the switch there is a 2 way that powers either the "Instrument Lights" [gauge lights] or "Ignition Lock Light" [controls] - a dome light was standard on panel bodies only but may have been optional for the pickup cabs, or yours was added by the dealer or original owner
Bill
