Question about car air conditioning?

I have a 1997 astro van with the rear air blowing ice cold. The high switch for the front does not blow out anything at all. When I put the switch on low or medium, it barely blows out at all. Whatever position I put the switch on (i.e defrost, feet, face,) it does nothing but blow extremely low. The rear defrost, which is another seperate switch to control, blows out very cold. What is the problem?