Defrosting Button Only Activates Under Dash Air

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