some linkage came off and I have it but don't know exactly where it goes to enable the defrost to work.
defrost doesn't work on 1990 Chevrolet 2500 Pickup
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.
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.