The only way you would see power at the door switch is if the circuit is complete & you read it back through the bulb or buzzer. The door switch simply provides the ground. Positive power is constantly fed to the bulb, check if there is power at the socket, on a '73 the fuse is cavity #3, same as tail lights so I doubt that's the problem.
I seem to remember problems caused by a time delay relay, your car have that?
