When heater is on the windows steam up

Suspect a leaking heater core. The clue is the film on the glass will be kind of oily when you try to wipe it off. If it really is just water vapor, it will evaporate on its own overnight. For that, suspect the AC condensate drain tube is blocked. The water that is supposed to drip onto the ground is being vaporized and is condensing on the cold glass.

An inoperative AC system will also cause condensation on the windshield. The system is supposed to run in "defrost" mode to remove the humidity so it will not condense on the cold glass.