2003 Buick Century Heat/Defrost

I have a 2003 Buick Century with a dual driver/passenger temp control. I cannot get heat to come up to the windshield on defrost. During the summer the air conditioning blew hot. When I did not have heat last winter I changed the thermostat. The car will generate heat. When air blows up to the windshield the passenger side gets more air than the driver’s side. I have heard about a door unit on Car Talk but am not sure what it is called or where it is located. Is it operated on vacuum, wire, or electrical wire? Do you have any thoughts?

The door you’re looking for to repair is called a blend door.

It has a small (electric, I believe) motor controlling it.

It is located in the ductwork and is a time consuming job.