Vacume line diagram for 2001 dodge grand caravan looking for the line that runs to the ac / heater,

It sounds like vacuum is not getting to the vent, heat and A/C controls. Find the vacuum can. It may be under the hood on one of the inner fenders. Trace the lines back to the vacuum block at the intake manifold or fire-wall, and also back to where the line goes thru the fire-wall. Check the can for leaks or obvious holes. They used to be just a plain metal can, but it could be a plastic ball. Check the lines as you go for breaks and rub-thru's. Move the controls thru all settings and listen for vent door movement. (Run the engine momentarily to build vacuum in the system.) Worst case is a bad vacuum motor or air flow position selector switch.