My 1999 Pontiac Grand Prix’s fan came on last night while the car was parked in my garage (and the motor was off)! The controls inside the car were ineffective and, as I couldn’t figure out what fuse would deactivate the fan, I disconected the battery to stop the fan. What could be causing this?
Sounds like something happened to the relay that activates to run the fan on high speed. That’d be the first thing I check. If that’s not it, I’d take a look at the control head.
There is a blower control module that controls the blower. If it sticks or malfunctions the blower will run continuously. Check your owners manual for fuse locations. There is most likely more than one box with the blower fuse. Mine is under the back seat.