The socket grounds the bulb, the lamp assembly grounds the socket, the cove panel grounds the lamp assembly, and the cove panel is grounded by spot welds that hold the body together. The body is grounded at the fire wall bu three flat grounding straps that connect the body to the motor, the motor is grounded by way of the negative battery cable.
If there is corrosion, rust, or a missing connecting link your tail lamp or license plate lamp it will not light up. After fifty years of exposure to the weather I can almost guarantee it is a bad ground, somewhere.
To find bad grounds in my shop I used a ten gauge twenty five foot long wire that had an large battery terminal size alligator clamp at one end and a smaller alligator clip at the other, that I used to touch things or attach the wire with the clip to a part to assure that I got a good ground back to the battery. If it lit up or other wise worked I knew I had a bad ground. It was a just matter of checking the connections back to the battery to find out were the ground was bad from there.