The car windows fogging on inside have become a common phenomenon during winters or sometimes in summers. Basically, the moisture content in the air becomes condensed and gets down on dewpoint, which causes it to be foggy. The dewpoint is the air temperature in which the moisture content gets turned into tiny water droplets, which gets foggy when vaporized due to a rise in temperature. So, if the temperature outside is too cold and you have turned a heater inside your car, it is more likely to make your car inside foggy, leading to the windows becoming foggy as well.