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!

My 73 dart sport does the same , blows out ac vent under dash only .I never drove mine in rain so i did not ever fix it but there is a vacuum switch mounting on the back of your heater control that could be bad, also could be vacuum hose leak or is it getting vacuum from the back of the intake manifold .I wa s always goinmg to fix mine but never did .You might just be able to switch hoses around at the valve slso to get defrost , hope this helps

Thanks kt340sport. However, I do not have any intake manifold vacuum connection,so it can't be that. Today was one of the few days where I'm glad I don't have a glove box, easy to take a peek underneath there. Phew, what a mess! Two vacuum lines were disconnected, and didn't have any suction at all anyways. There is a two pronged switch disconnected as well, with the prongs perpendicular to one another. Have no idea where that goes. A lot of the rods and such in there connected to the diaphragms were very stiff and sticky. I moved one rod underneath, near the floor on the passenger side. That seemed to help, with the defrosters (and the floor heater vents) actually having air come through them. But now, of course, my A/C vents now have no air because of the shift. I didn't find any dry rotted hoses (yet).

I really ought to pull the dash and get everything fixed underneath there, since there is one nasty leak that drips onto my foot when it rained pretty good also. Are there any diagrams for all of the vacuum hoses? A service manual I'm guessing? Found one cheap, but haven't pulled the trigger yet.