why does my A/C on blow through the defrost? on 2000 Ford Mustang

Why does my A/C only blow through my defrost no matter what setting its on? it blows hot and cold but only through t he defrost.

Asked by for the 2000 Ford Mustang
there is a small vaccuum line under the hood back by the firewall in the center that may be off or broken.

1 more answer
maybey becuse the battery is low cuse my did thate 30 miutes latere just cahrge battery man