Help!!

I have a 99 Chevy Tahoe 4wd, 5.7 engine. I just noticed when using my a/c or heat, the air comes out the defrost vents no matter where I turn the vent selector knob to. Is the control knob bad or do I have a hose disconnected somewhere? Thanks!

I think that one still uses vacuum to control the mode doors. Check for a vacuum leaks under the hood.

It is the motor that controls the air direction located at the bottom of the floor vent on the gas pedal side 3 screws hold it on I think, then unplug wiring harness quick disconnect and replace the part DORMAN Part # 604107 {#89018395, 89018681}
Controls A/C Blend Door; Temperature & Mode Door; Main Unit; w/AC
You can get it at most auto parts stores for under $50

The blend door and mode door are two different things.

They are two different things. This system has 3 doors they all use the same motor part number. At least on my 99 Tahoe.