is there a door to direct air flow not closing completely?
2004 santé fe has cold air coming in at the floor on passengers side. heat is on on 2004 Hyundai Santa Fe
poss bad door actuator
If there is you should find a lever to control it and it's operation would be noted in your owners manual. Does your SantaFe have a dual climate control system? Restrictions in the heater cores on some cars can result in some unequal heat distribution.