Frost buid-up inside of windows

What would cause frost to build-up inside the windows?:screwy:
Anybody have a clue on how to repair it.
Have good heat inside.
It's a 97 Hyundai Elantra GLS

Usually it happens if you get too much water in the air inside the car. Make sure when running the heat you have it on fresh air (pulling air from outside the car instead of recirculating) and you can also turn the a/c on to help take of the water out of the air (yes, even on hot settings you can use the a/c--it won't get cold don't worry it will just run the a/c condenser).

Things that might be causing the problem:
- Coolant leak in the heater core. Would be pretty evident by the smell, but if its slight, and you have car freshners or other smell wells, could be an issue. Can verify by keeping an eye on the coolant level in the overflow bottle
- Leak coming from window gaskets or other source of outside leak. Check the carpets top side and underneath for dampness.
- Fresh air vent (as addressed by peachferrari). Might want to check the operation of the vent/recirc button to make sure the flap is moving