Trailblazer vents wont change from defrost ,to your feet,to your face etc. 2004 just had air compressor replaced if that matters.

Defrost is default mode. You have an electrical/vacuum/blend door motor problem causing this issue.You need the system checked by a pro to determine the failure

If you're looking for vacuum hoses for the HVAC system, there are none. The HVAC air/temperature is controlled by doors and electric actuators. You may have a stuck door or bad actuator.

Any codes? Fuel filter and air, dirty throttle body.

2 possible problems, #1 is the control switch on the dash has malfuntioned(less likely) , #2 is the motor under the dash that changes the blower position is not working properly.

Please rate my answer ............Thanks

Heater cable sounds like it's stuck. Follow the heater hoses and lub the assembly.

Sounds like a compressor and/or oiling issue. The diagnosis and repair requries special tools and equipment, take it to a shop...

first check to see if you have power going to the actuators when you switch the switch to it location if no power replace controller if you have power and no movement from actuators replace actuator. if actuators move remove hvac box and check for broken blend doors.

Typically, you have a vacuum issue, air flow system is designed to FAIL in the defrost mode, so no vacuum, doors are spring loaded to blow out defrost ports.

sounds like it is vacume operated check to see if all vacume hoses are connected to the control switches and vent controls. i suspect somebody has caused a hose to disconnect tampering under the dash

Inside the ventilation system there is gates that open and close to direct the air flow. These are operated on vacuum on most vehicles. The hose for that perticular gate may have disconnected somehow or has a hole in it. I suggest you take the heater controls out and see if there is a plug with a series of colored hoses on it. it so you'll have to trace each of them to see which one is the problem. Now you may be able to get some help from you local dealers mechanic who just may know the color code for that gate. (which would be a bonus) Good Luck