Still blowing air

You need to get out your shop manual (you DO have one?) and study how the controls work

You can download a 72 manual here, but I DON'T KNOW if anything changed for '73

Page 24-50

http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download

Offhand, I'd say you have

a bad vacuum control

leaking/ disconnected/ mis-connected vacuum hoses

a bad panel control switch

something AFU in the airbox doors

One thing you can do for a temp fix is bypass the heater core, or put a hand operated valve inline with one hose