Why do your windows fog up when heater is on?

most of the time it is because the heayer core is leaking allowing steam from the coolant escape to the interior of the vehicle.





That is not the correct answer. If a heater core were leaking, there would be antifreeze on the floor of the vehicle and you could smell it. The condensation in from natural evaporation of dampness in the car. In the Winter this often happens because there is water in or on the floor mats from snow and ice. Turn the AC on and turn UP the temperature like you were turning up the heat. That will take the moisture out of the air in the vehicle and help dry the floor.