my problem is that air stays on defrost wont change to heat or vents. Any idea?

Vacuum leak under hood on passenger side.. Really small vacuum tubing from rear of intake to heater A/C case going through firewall is broken or unhooked.. LOOK around for it!.. Vacuum controlled mode selector for this system and defroster is the default setting if vacuum is lost.

you have a vacuum hose leak.raise the hood,look next to firewall where the smaller hoses are,passenger side.start truck,turn a/c on and listen for hissing sound.feel around the hoses with your hands.there are repair parts at most parts places,cheap to fix this with.check all vacuum hoses for splits,dry rot,disconnected.may also be under dash,but most of the time,it's under the hood.when you have a vacuum leak,the blower motor will default to the defrost mode.post findings!