Air Blowing Only Out the Defrost Slots

[palallin]

On my New-to-Me Tahoe, the blower only puts air out the defrost slots on the dash no matter the position of the selctor dial. Am I going to have to remove the dash or some similar major surgery? I had a similar problem on a '98 Explorer, and $.69 door pivot would require installing a whole plenum box (@ about $400, not counting removing the whole dash and much of the forward interior).