You might have a gas leak in your front engine compartment. The heater pull in air from the front and therefore if you got a gas leak it will pull the smell in get it check asap.
the heater core may be plugged up
heater core
It's leaking somewhere, possibly in your heater core.
The heater blower may not be working properly. Without the heater blower working, the air will not come through the vents.
Antifreeze will be leaking in the floorboard on the passenger side, there will be a smell of antifreeze through the vents.
Generally, with that smell- the heater core has gone bad.
The possible explanation for the smell may be that the heater core in the vehicle has a slight leak, therefore the smell of evaporating antifreeze will come through the vents until it is all dried up. Solution is to take the car to a garage and get the heater core replaced.
Chances are it is a busted heater core. It your window has a fog on it when you turn on the ac or heater.
Check for a line leak to your heater core or a leak in the heater core it self. Usually it is under the dash on passinger side.
dryer sheets in the heater vents may help
More than likely the heater core is leaking.
bad heater core bad heater core