98 blazer w/auto air delivers air out dash vents only

My 98 Blazer LT w/auto climate control system will only deliver air to dash vents.Fan control and temp control work fine whether in auto or manual modes.The mode knob whether in auto or manual mode will only deliver the air to the dash vents.This problem started a few days ago with all three control knobs in auto mode the heat was coming from dash,generally heat mostly went to the floor when in auto mode.Had to manually put air flow to floor or defrost,worked fine for a day or two that way.Now air is all delivered to vents in dash no matter where I put the mode knob. Any ideas on where to start to fix this problem would be greatly appreciated.

Check for good engine vavuum from engine to vacuum ball and on to heater controls.
Lot of plastic line and rubber couplers that break or get unpluged.