Heat or a/c on-only blows out window defroster vents, none of the other vents. ???

There is a mechanical mechanism with a physical tunnel switching plate under your dash. Most likely in the center under the HVAC control panel, or it might be offset to the passenger side of the dash under the glovebox. Well depending on your car, if your controls are digital or manual. Manual meaning you have to turn your dial and you feel the linkage move the plate. Or digital you are just pressing buttons and the computer does it by activating solenoids. Your door is stuck for some reason, once again depending on your vehicle. It could simply be stuck from a sticky moisture that built up, or more likely the computer's mechanical components have burnt out. It could be a resistor or a capacitor that is dead and therefore not allowing your HVAC system know what is going on. If you work on vehicles I suggest you might take a look first at the physical switching box which ports the air flow in the various directions. Check to see if it can move with ease. If it does, or you don't work on vehicles I suggest you take it into a dealer to check it, because if it is digital and not manual it could be a few different locations of the problem to find it.

Thank you Brainybear72. It is digital. I'll be replacing the HVAC Programmer as soon as I can find one - (well, actually not me, have to take it to a mechanic, not the dealership) Is the "Programmer" what you are talking about?.

It may be. There are a few possibilities. There was a vehicle I was working on a few years ago and it had a 'digital drift'. I would set the door correctly and then over a period of a few days the computer would slowly tell the location off, until that vent panel was again in the wrong place and "stuck open" as you have right now. Only testing can truly determine where the problem is. I can only best guess.

Thank you Brainybear72-I appreciate your responses! Frustrating it is. Testing as in the computer read-out testing? I have tried that. It's been "confusing" also-troubles reading I guess. Once the HVAC Programmer is replaced, I can take it back to the dealership to see what the computer shows, if anything. Like a needle in a hay stack. Thank you.