After the fog has cleared try this test
At idle
Select Panel or Floor
Feel if any air is coming from Defrost vent, it shouldn't
REV engine while feeling for air at Defrost vent
If you get some air flow as Vacuum drops in engine(REVing) then you have a leak in the Vent Vacuum system
This usually occurs on the vacuum line that runs from firewall to Vacuum Reservoir "ball" in engine bay
Its a black hard plastic vacuum line and it can crack or melt, near exhaust pipes
Could also be the Reservoir is cracked and leaking
The Vent doors use vacuum "motors" to hold vents in selected position, Defrost is the Default position, the No Vacuum position
The Engine is the vacuum source, vacuum is high at idle, low while accelerating(REVing)
The vacuum reservoir is used to store vacuum, like power brake booster, to hold vent doors in position regardless of engine vacuum
If there is a leak then vacuum level at the vents follow vacuum in the engine, so if vacuum in the engine drops Defrost vent gets air flow because vacuum is low
The Blend door is the temperature control door, it is electric from 1995 and up, not vacuum operated