1988 k2500 defrost problem

[kinger62]

Hi, my 1988 K2500 will not blow air through the windshield defrost vent. The actuator motor opens and classes the proper doors to get air on the floor and through the dash vents. When I push the button to get the air to defrost I can feel the air blowing on the door windows through the little side vents but I get none on the windshield itself. Does this mean the vent is blocked? Any suggestions for troubleshooting? Thanks!