98 tahoe heater problems

My tahoe heater doesn't direct the heat to the windshield when I have the defroster on. It works well in all other positions and blows out good but its seems not to be switching over to defrost. I have a '05 gmc 2500 that does the same thing too. What causes this? Thanks..

My tahoe heater doesn't direct the heat to the windshield when I have the defroster on. It works well in all other positions and blows out good but its seems not to be switching over to defrost. I have a '05 gmc 2500 that does the same thing too. What causes this? Thanks..

Check mode door actuator positions and linkage.
Should be the one by the gas pedal.

If no problem there remove actuator motor and see if you can move the door to defrost position by hand.