My son's '98 K1500 will blow out of the defrost, ac vents and floor vents all at the same time no matter what setting you put the controls on. The heater blows hot and the ac blows cold, but out of all the vents at the same time.
Does he have a broken door?
Also how hard is it to take out the whole box assembly? Can we just take it out and switch it out with one from a salvage?
I ask that because he had a broken hinge on a heater door on his old '86 Chevy he had several years ago.
Thanks in advance for your help on this.

 

Try the control unit itself . It may work with the other two settings but that one may be stuck.