1990 Pontiac 6000 heater works great but defrost won't turn on. Switch light works? any thoughts? Thank you.
Sounds like a mode door actuator problem. Not sure about your model but older model cars used vacuum to move the doors. You could just have a vacuum line loose. I will do some digging.
Yes it appears to me that yours is vacuum controlled. Look for vacuum leaks. Should be a vacuum hose coming off you intake and going thru fire wall to air controls. There should be a 6 way(line) diverter valve behind the dash that controls which doors open and close. Inspect it for vacuum and vacuum leaks. Hope this helps