The blower fan for my 2004 Buick Rendezvous heater and ac does not shut off. It also does not change the speed when advancing to the highest setting.
Could be the switch is broken, or a relay that’s sticking, or a circuit board or connector problem. Find a shop w/access to the car’s wiring schematic. They’ll have to test it point by point following the circuit from the switch to the motor until they determine where the problem is.