GM-GMC: heater problems, chevy silverado, plastic vacuum

Question
i have a 1999 chevy silverado 5.3 my heater wont switch from vent to defrost what and where do i start checking please dont have much extra money now due to xmas so close appreciate any help

Answer
In the engine compartment look for a disconnected or broken vacuum hose that feeds the heater switch. On some trucks there is a plastic vacuum reserve tank that holds the vacuum for the hvac system. On a auto heater/ ac system you may have a mode actuator that is not working. The actuator is inside the vehicle under the dash. Hope this info helps. GOD bless