Dome light won't turn off

Hey guys, I can't get the dome light in my '70 Duster to work properly. If I put a bulb in the socket it stays on no matter what I do with the door switches (doors closed or open) or the headlight switch (which I have replaced with a new one from Checker Auto). I've carefully studied the wiring diagrams but can't figure out where in the circuit the problem is. Has anybody else had a problem like this, and how might I fix it? It would be nice to have a working dome light so I can see inside my car at night.
Since you've replaced the headlight switch, I'd check the door pin switches. They act as a switched ground. It could be that the switch is somehow crapped-up, or one of the wires has found its way into contact with a chassis ground point (fallen off the switch, worn through insulation, etc). I'd have to look to be sure, but I think they'll just screw out.