Trailblazers do not have vacuum lines to control the heater doors/box. All the doors are controlled by servo motors that use voltage regulated by the heater control panel. If 1 of the door servos is not working, then you could have air leaking to all vent's (defrost/vent/floor). Does your blower fan seem to change speed when you turn it up and down? Does the air change from defrost to vent to floor when you change the switch? If not, than you have to check for power to the motors on the doors and blower. Dash board dis-assembly will be required to check/replace servos.