ac blows thru deforst on 1998 Ford Mustang

my ac blows cold air but it only blows thru the defrost vent what is wrong with it and what do i do to fix it

Asked by for the 1998 Ford Mustang
check under the hood by the driver door and make sure the small vacum hose is hooked up and not broke cause my buddy has a 97 and that is what was wrong with it