Whenever I have the heat or ac on in my car it smells fine until I stop. If my car is not moving and the heat or ac is on it smells something like exhaust. Sometimes it is stronger than other times. Not sure why it would only happen when car is parked or stoped.
STOP DRIVING IT until you get the exhaust leak fixed .You can easily be overcome by the carbon monoxide.While driving the air moving under the car is sweeping away the exhaust,when you are still the exhaust is leaking into the passanger compartment.