ir only comes thru defrost vents why?

blend door actuator could have failed,also check vacuum lines down by gas pedal make sure you didn't kick them off.

Defrost is the "default/safety" setting for the HVAC. Your blend doors have to be powered to a vent or floor setting. You might also wanna check the 1way valve on the passenger side of the intake where the HVAC n cruise vacuum lines tap into the intake. If the 1way valve goes bad, the HVAC will default to the defrost, however, usually only on acceleration tho.

I had the same issue on my 96 Dodge Ram 1500 (5.2) I replaced both check valves and still had same issue. I got in my truck and started it (without a/c on) and listened for any hissing type sounds. Found it! Behind the control knob panel where the vacuum lines plug in. In my case, i unplugged the lines and saw a plastic geared rectangular box that controls where the air comes out....it had slipped of the gears and the clear rubber part that plugs the lines into it had backed off some. (This caused the leak)

I have a 96 b 3500 conversion van/rv. The ac blew from the defrost only. This site helped me to check out all vacuum lines. The tiniest of a hose was my culprit. It was the line that went into the fire wall from the engine block. Replaced it using rubber connectors from local auto store and the ac worked perfectly. Amazing that the smallest vacuum leak can be the difference in the air flow door control.