Map light fault - HELP!

Hello FABO friends, Cruiser here. New to this forum. Anyhow, could anyone help with a weird failure mode that I'm experiencing in my very stock 74 Duster? You can view the car "Bob's 1974 Gold Duster" in the member rides section. Anyhow, it's extremely stock and the under dash wiring hasn't been messed with. When I turn on the map light switch, the map light doesn't come on. The bulb is good, and all wiring looks good. When I open the door, the dome light comes on as it should, as does the map light. But then if I activate the map light switch, the map light goes off. At night, driving with the lights on, the map light will not come on when I activate it with the switch. But if I rotate the interior lights rheostat to full bright and activate the dome light, the map light comes on, and again if I activate the map light switch at that moment, the map light goes off. Something is bass awkward here. For normal night driving, turning on the map light does nothing. Any ideas here?