Welcome to the forum
Blend door is electric and is for hot/cold, it has nothing to do with the direction the air comes out, i.e. defrost or vent.
Vacuum controls the direction of the air, defrost is the default, so off or no vacuum = defrost selected.
Vacuum comes from the intake manifold, it goes to "the ball", a vacuum reservoir, it has a check valve like power brake booster does, so it holds a vacuum even when engine is off.
There will be a vacuum line from "the ball", that runs thru the firewall and comes out behind glove box usually a hard plastic line, they crack.
First thing is to find that vacuum line that runs thru the firewall, pull it off the vacuum reservoir.........and suck on it, lol, yes suck on it, if it holds vacuum then problem is in engine bay, "the ball" or hose to intake manifold.
If it doesn't hold pressure then problem is in that line going to firewall or inside the cab.
If possible have someone inside the cab while you BLOW thru that line, maybe they can hear the air escaping.
Last edited by RonD; 06-22-2016 at 10:19 AM.