The cargo pit overhead lights on a 767 are 28VAC powered via a step-down transformer from a 115VAC circuit breaker.
The light installed on the door receives this same power. We had an aircraft with the door light inop. A new bulb was no help. Checking for power and ground on the bulb wires revealed that power was present.... but no ground.
All door control wires and light power use one harness.
On this particular aircraft, power used pin 20 and ground pin 21. We checked continuity from the door side receptical to the light itself. These wires were good. The aircraft side of the harness enters the airframe at the top rear section of the door hinge.
The harness does "not" have a corresponding disconnect plug on the aircraft side. It's "hard wired" down to the door light control box on the aircraft's right side-wall. We started gutting both the overhead and side-wall looking for the ground stud associated with the pin 21 wire.
After an hours' worth of (needless) work, we opened the door disconnect plug. There was no wire connected to pin 21. A long pair of needle-nose retrieved this....
The re-pin and insertion was quite difficult as the harness elbow did not allow much room. We were lucky we didn't bust any of the other wires.