when heaater is on blows cold air out of the passenger side. what would cause this?

That car has dual settings for driver and passenger ... my first check would be in the temp control.. With the engine on and all other noise abated ( radio,CD and blower motor on low speed) move the temp setting on just the passengers side ... if you hear something under the dash the the temp sw is probably OK but the actuator is not shifting the mixer door.