If air only blows from the defoger vent how do i fix it

the ac is the best item to use to demist windows as it removes the humidity from the cabin air
make sure that the vent control is set to re-circle cabin air and not get air from outside
run the ac with the heater control removes fogged up windows quickly
If there is a smoker in the car , clean the interior glass with suitable c glass cleaning agents as the oil from the smoke makes defogging extremely difficult

Might be the control switch, but that is unusual. Most likely it is the vacuum hose going to the damper controlling the defoger. If it was the vacuum line to the control, then it would blow defog all of the time.

warmth comes from the heater in the car
if it is climate control check the climate control module
check for heater control in the vent settings as the hot water tap may not be turning on

sounds like the mode switch has gone bad and needs replacing.the mode switch is the one that changes the direction of air flow from defrost to dash to floor. if nothing has blocked the vents to the window then this is what it sounds like.

Your air-conditioning system may be low on refrigerant, and/or may have a leak.

ive had three of these trucks more than likely control pannel is bad an early 96 dashes were the worst one mined sucked to

Sounds to me either you accidently disconnected a vacuum line (controls the internal vent doors) or its over charged. But my bet is on the vacuum.

i believe the controls are vacuum operated. when changing the battery, you most likely, however inadvertently, broke or disconnected a vacuum line. when vacuum is removed, the system defaults to the defrost postion to make sure the windshield can be defogged.

that switch controls a vacuum operated door that lets a/c or heat thru.when accelerating,the vacuum in the engine changes so it sound like a vacuum issue.take apart the center stack and there is a vacuum actuator in there connected to the flap door .i have seen those have issues with the metal tab broken away from the plastic that moves it.is the a/c piping cold under the hood where it goes into the heater a/c assembly?the compressor should be cycling on and off and the pressure tested.