Open fresh air vent and set airflow setting to windscreen only or windscreen and legs with blower fan running at moderate speed.
Make sure that the fresh air vent and other vents are opening and closing properly.
You also can use heater or AC temporarily or Windows slightly open for de fogging, but fresh air vent (warm or cold as required) is most effective.
Use 82C thermostat valve, for both seasons. Good for engine and heater.