all of sudden my air flow stop on windshield glass on 1998 Chevrolet Venture

all of sudden my air flow stop on windshield glass so i can't drive in rain. if i block (close)all vent then i get very low air flow in windshield. how can i fix it

2 answers 2 comments
Check for any broken vacume lines that supply vacume to HVAC controls. Vacume is supplied from engine intake manifold.
thanks a lot
Sounds like selector switch not switching, will it switch from vent to floor and back? Anything fallen into defrost vent? Check ebay for control panel at a good price, if needed. No vacuum defalt should be defrost!
thanks a lot