Temperature gage goes to cold and my windows fog up?

Heater problem
1990 Toyota Corolla 4 cyl Wheel Drive Type unknown automatic 194000 miles

I live in a place where it gets to be -30 and if I let my car warm up for 20 minutes the heat is nice and warm but as I drive around my temperature gage goes to cold and my windows fog up and the air is cold. It get really cold when I drive on the highway at a faster speed than just driving around town. My dad thinks that it is the thermostat. He has already put some card board in the front. But that hasnt seem to help. Do you think that it is the thermostat?

thanks.