On my 70 and 71s I just added a diode pigtail across one of the open, left over connectors in the main harness. The key in buzzer then works as a headlights on buzzer too, when the door is open. I think the wiring diagrams show where to plug it in. It was an option that came on e-bodies. One of my pigtails had a part number on it too.
Before I put this in, I used to leave my parking light on all the time. I like to drive with my parking lights on.