[supereal]

The switch is inside the socket, and is a mercury (liquid metal) type, not to be confused with the car brand. The accessory under hood light is the same type. In many cases, the switch part has failed long ago Most just connect in the license plate lamp lead inside the trunk lid. The idea was to come on when the trunk lid or hood was opened. This could lead to a "parasitic" draw if the light stayed on, and many were connected directly to the license light wire so it would come on only when the lights were used. This accessory was probably devised to allow the dealer to get a bit more money when the postwar cars came out.