Why does my windshield get foggy when I turn on the heat in my car?

You may have a bad heater core. You can check this by seeing if the carpet is wet on the passenger side just below. The dash also check antifreeze level. If all is good then try your climate control knob turn it down some little at a time

The a/c has a setting where it blows air near the window. On the dashboard check to see if there are air vents near the windshield. If there are, set the air to cool and set it to blow through those vents. Don't ever use hot air in front when it's cold. Blow it through the bottom. :)

If it leaves a oily film that doesn't disappear in a short time when you wipe it off with a clean towel, it's a leaking heater core.

Turn your ac on. You can leave it on heat but just push the ac button and the compressor will turn on and it should go away.

Open your side window half inch to an inch.

if oily window...heater core

do not turn on

you cant its condinsation in the air.