On the older cars like my 67 the D terminal is grounded when activated through the switch to turn the dome on. It is my understanding that some of the later cars actually have a separate ground wire connection. I would guess that Ma had trouble with this on some cars
The circuit goes fused source.........through the lamp........to the yellow wire buss. The yellow wires are all hooked to door switches. When you open a door, the switch grounds that yellow and turns on the dome Same deal with the HL switch, when you twist the knob, it switches the yellow to ground.
The bulb is not grounded. It's a dual contact bulb, and the filament is hooked to the two bulb contacts