The heater works but the defroster does not on a 2000 Grand Caravan - does anyone have a solution?

When you change the settings on the climate control system (i.e. from floor to defrost to bi-lev) does the panel hiss? You should be able to hear a hissing noise as vacuum is transfered from one part of the system to another. If you do, check the air door and control for the defroster. If you do not, check your vacuum supply. Check the shop manual at your library. The shop manual goes into incredible detail and has problem-solving flow charts.