The slide is attached via cable to a mechanical door in the heater box which shuts off the flow of air. If the slide seems to move without any resistance then the cable may be broken. If you can hear the door move in the heater box then it's possible the foam seal is deteriorated and no longer sealing.
The kick panel vents and dash Astro vents are also sealed with foam rubber flaps. These vents will leak if flaps are deteriorated.