When the heater is on exhaust smoke is coming in through the air vents.

Heater core is leaking and will need to be replaced. It's steam and most likely has a funky smell.

If the "smoke" you see is usually worse on defrost and has a sweet smell to it , leaving behind a fine film that has to be cleaned off, it is your heater core that is leaking, spraying a fine mist of antifreeze in the air. Check the center floor area around the console for signs of moisture or have the cooling system pressure checked.