heater blower want shut off

If this is happening in very cold temperatures, look at this technical service bulletin from GM: Condition: Some customers may comment that the blower motor is inoperative or continues to run after the vehicle has been shut off. Cause: These conditions may be caused by a blower motor control module failure. This failure may occur when the vehicle is operated in ambient temperature conditions of -23°C (-10°F) or below. Correction: Important: Replace the blower motor control module if the vehicle experienced a blower motor control module failure in extremely cold ambient temperature conditions.

it sounds like a bad heater relay