In later A-D panel-body trucks (maybe 48/49) with rear doors, a solution similar to the metal-coil & hole near the hinge is what the factory used to route a wire into the rear-light-housing mounted on one of the rear doors.
Prior to that (maybe up to 1948/49), two metal mating-contacts were used on the door and the cab frame. Contact/light-on was made only when the door was closed, and, no-contact/no-light when the door was open.
