The electronic heater comtrrol unit are behind the front footwells ahead of the plenum.
You should also check the actuators that open and close the air vent doors in the heater box. The heater box is under the bash in the middle to the passenger side. Every vent door has a small motor that closes and opens them called an actuator. They do fail. You can also make sure the doors are not binding for any reason. You will have to take most of the heater box apart to get to the air vent doors.
I don't have a diagram of where the doors are, we don't have them for any car actually, I am not sure a factory manual would have them either.
Once you remove the main cover you should be able to see most of the doors as they are in the ductwork path.