If you mean in the tail light cluster, the bulb grounds through the metal strip the bulb screws/bayonets into. The metal strip is then grounded to the connector. A common failure is for the claws in the tail light cover to deform and fail to hold the metal strip tightly enough to keep good contact with the 12V spring terminal the tip of the bulb should contact. IIRC the only solution is to find good tail light covers.

Sixto
87 300D