Dual heater does not work on driver's side

Any ideas for fixing dual heater that does not work in any mode for the driver’s side of our 2005 extended cab Chevy Silverado?

Passenger side operation is perfect.

Possible faulty blend door (might be stuck).

Thanks ever so much… Just how do I check the blender door???

The blend door and motor is in the ductwork not far from the blower.

So, roughly between the stereo and the glove box. Some are closer to the glove box.

You have to remove some of the dash.