Defrosting Button Only Activates Under Dash Air

As the rain continues to pour down on the Northwest, the windows on my Dart are fogged up beyond belief when I drive. I have to crack my window and try to blast the cold heat open just to keep the fog at bay. I do not have rear window defoggers, just front ones (although I'd like to find one...?). When I press the defrost button, air does come out, it just comes out of the A/C/Heater vents under the dash. Everything else works (low to high, heat to max heat, ect.). As far as I can see, the hoses for the defrost vents aren't torn or have any holes. Something must be routed wrong, I just do not know what to look for. Any help is appreciated, and hopefully can fix it ASAP!