2003 Chevy Astro not working properly

I have dealt with that situation a lot of several cars. More than likely yours really does not have vacuum lines, instead they switched to electrical motors called actuators. The problem could lie in a bad actuator or faulty climate control not sending the right signal. Kind of expensive to guess and check so you really need to have an experianced auto mechanic to check it to make sure which part went bad.

This vehicle is equipped with vacuum actuators for mode selection and electric only for temperature.
When there is either a partial or total loss of vacuum, the system will default to either defrost or on some vehicles, heat. Acceleration can also have an effect on the doors for the same reason.

Look under the hood for a small black plastic hose coming through the firewall. Follow it to the vacuum source looking for any breaks, cracks or leaks. Also follow any other lines that branch off because they may go to a vacuum reservoir (storage tank) and a leak there will cause the same problem. Using a vacuum gauge to determine if you have full vacuum at any given point is the best way to find it. You can cut into the line anywhere and splice it back together with a piece of vacuum hose. Find the broken line, you found your problem.