heater problem on 1999 Chevrolet 1500 Pickup

my vent wont switch to defrost or any other position all functions work ac heat just wont switch from vent where are actuators located at on this truck

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2 answers
electic motor for heater door i can not think of name of part bleed actulater motor 39.95 atautozone.com go to heater parts
Typically the direction of air flow (defrost, dash vents, floor) is controlled by a vacuum motor. Check around under the dash for broken or disconnected vacuum hoses....it may help.