Usually the ground is shorted to the socket. The switch at the doors grounds the one pole on the bulb. So if the bulb is off and you ground the socket and it lights you must have the ground side of the bulb making ground.
Did you all know that constant power to the dome is the reason they discontinued the so called 71 vent pipe in the trunk. There was a bulletin on that pipe in mid 71. But no recalls were ever done. Fire starter. The CIP pipe
