If your Astro is blowing on DEFROST only the reason is a bad vacuum line, or in my case multiple vacuum lines! In order to find the "boot" on the manifold, you must know what you are looking for and as they say a picture is worth a thousand words. I read all the posts and could not find it anywhere. It's located on the driver's side of the engine about half way up the motor. It could be hidden by wires, and it's horizontal and looks like a little spark plug boot, but it faces the engine so it's really tough to spot.
Once you find it, it can even be tougher to get your fingers on it. I replaced the hard plastic with 1/8th inch rubber hosing bought from Advance Auto Parts for about $4.00. then I ran it across the engine to the engine compartment and it ran to a "T" fitting behind the A/C condenser on the passenger side of the engine. All of the hard plastic lines were broke, so I ended up replacing all my vacuum lines with the rubber type so hopefully it will last for years. Good luck, I hope this helps!
