1996 Ford Explorer Snow blowing inside the car when defrost

My Son has a Ford Explorer which he has had for 2.5 years. The first 18 mos he lived in the Seattle, WA area which had very little snow.
He is currently going to college in Moorhead, MN, which has received a lot of snow.
He did not have any problems until the latest snow storm and -20 to -30 degrees below zero weather.
The problem now is that snow blows inside the car through the defrost and heater. Of course when the car warms up the snow melts and now he has a lot of moisture in the car.