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.

2 answers
there is a small vaccuum line under the hood back by the firewall in the center that may be off or broken.

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