back side, ( firewall side) of the intake manifold is a vac line that leads to the firewall, it then runs across the firewall then down the pass fender top down to the battery area, some have a T some dont, but at that point it will run into the wire harness leading into the cab.
check that line for a break. it often gets busted by the battery when its changed. it powers the air flow direction.
if that line all checks out. theres a box under the dash that line runs to, that box can fail.
or worst case is the plastic linkage on the hvac unit in the dash broke. thats no fun to fix at all.
