I have a 1964 Buick Electra with factory a/c. My heater fan won't shut off when i turn the key off. More precisely it won't shut off if the A/C is on. If I set it to vent or heater it will shut off with the heater fan lever and I can adjust the fan speed. If I turn the fan lever to on in any setting it stays running even when the key is turned off. Any thoughts or ideas are welcome.