On my 71, the red wire goes from the fuse panel to each of the 3 lights (dome & 2 door lights). This is the hot wire. There is a black wire that connects to the other terminal at each of these three lights that goes to the door switches & the courtesy light switch. This means that the switches complete the circuit to ground. Have you checked the bulbs in the door lights? Corrosion at bulb sockets?
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