2004 trailblazer dash vents fail to blow heat. All other vents work normally. I could not locate a vacuum hose that I thought might operate the upper dash vents. what do you suggest? Thank you.

The selection door, is operated by 1/4" vacuum hoses. When turning the knob or sliding a lever, to select between floor, vents, or dash deck for windshield defrost, vacuum is created by the valve body and the blower will then blow thru the desired vents. When these hoses/lines are compromised, especially on a vehicle over 15 years old it seems likely, vacuum won't be created, and the last working position, is normally where it stays! Until the lines are replaced.
Good luck.

Have the vents checked. See if there's a leak or something block the air passage.

Yes a vacuum leak can cause the vent valves under the dash to not operate. To check this go from heat to defrost and see if you can feel the air flow change from out the dash or foot area to the window area. If it does change then it is probably not a vacuum problem.

The heater / a/c vents are controlled by vacuum,or electronic devices. What is happening in your Trailblazer is the Vent Doors are either stuck in a open or closed position thus only having air flow to the floor vents only..The actual vent door is located in the heater box next to the blower motor.

Look for vacuum leaks. System is designed to fail in defrost mode. so.... vac hose off, for example, would cause an issue like you describe.

Scan for bcm codes. Possibile door actuator motor inoperable. Also check for vacuum leaks under hood and dash

more likely a broken door in hvac case.with car running work controls and watch actuaters operate to locate good luck maybee a little challange

check vacc line from engine into fire wall. behind batt is where they normally crack and leak. possible from a leaking batt at one time. check black supply vacc line right behind battery

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

they are often operated by vacuum... look under the dash for any stray vacuum hoses...

Robert