2000 Blazer - No heat to the floor

Hi Everyone,
Looking for some advice here...I don't get any air flow to the floor when the control is set to the floor, it is deflected to the defroster. I get air
flow out of the dash vents when switched to the vent position. I am guessing it is a vacuum problem, but haven't got a clue where to begin the tear down. Anyone else had this problem? And what was your solution? Thanks in advance,

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