no air flow out defroster

Inside of your HVAC unit are what's called "Diverter doors and Blend doors" that are controlled by vacuum solenoids.

Your issue can be with either a stuck or broken door, a bad solenoid or vacuum line. You'd have to take apart the Unit to gain access to these parts. If you're interested in doing this yourself, write back and include your engine model. I'll walk you through it.

Check selector switch check for mix routed vacuum lines check door actuator on the heater box