Air Coming thru Defrost Vents

[84LX89GT]

chances are that if there's outside air coming in the fresh air flap door is leaking or damaged. There's a door that is vacuum operated (or cable/linkage depending on what year) that when you set your system on vent or A/C fresh air is allowed in, putting it in max a/c closes the flap to recirculate and further cool/heat the in cabin air.
The things i'd check is the seal around the steering shaft, see if any body plugs are missing (rubber grommets out of place, etc.) and worst case is take out the entire dash and heater plenum and check all the seals and fresh air inlet door.