Chevy truck climate control problems

Hello, I have a 1993 Chevy C1500 pickup. The problem is with the climate control. After the truck has been running for more than a few minutes the air stops coming out of the vents. Regardless of the setting on the dashboard controls, set to hot or cold, set to dash, floor or defrost? no air comes out of the dash vents, defrost or floor vent. You can hear the blower fan running full speed? but no air comes out anywhere.

I replaced the climate control dashboard unit, and nothing changed. Any ideas?