2003 GMC Sonoma 4WD

[Jimmie48]

The ac / heat only blows the air through the vents on the front of the dash, will not switch to the defost on the window / the floor defrost. The selector switch will shut down in the off position. The hot/cold switch works, fan control switch works, the noise from the dash sounds like its trying to change the direction of the air.

[MT-2500]

No vacuume will do things like that.
Check for good engine vacuum to heater controls.
Look for and find vacuum tank and check for vacuum there and check plastic vacuum lines from engine to and threw vacuum ball and on to heater controls.
Auto or manual heater controls?
If a vacuum leak under dash see if you can find it.