Your coolant is always circulation through the heater core, even when the heater is OFF, there is some leakage. This smelly stuff comes throught the defroster vents as well as the heater ducts, but with the A/C on, it is diluted! The heat and cold air come into the cabin through the same ducts.
ALL CARS sold in the US have heater cores. I lived in the Tropics for 5 years and actually had a locally made car with NO HEATER CORE, since the lowest temperature was 20C or 68F!