defrost doesn't work

can you see the linkage ? chevy had problem with that door that directs the air to the windshield and i have seen a few that stick there is a motor that controls that door.

Lots of times blend door problems cause the linkage to pop off, may have to do some disassembly-like replacing heater core-to get far enough to see problem. Need to verify door movement to allow linkage to not bind.