2009 Yaris with ICE on inside of car windows

[CTScott]

As I mentioned in another thread about severe Winter window fogging, excessive moisture inside of the car is likely to blame. This can be caused by something as simple as having live bodies in the car on a super cold day and not extracting the moisture with the A/C or by some source of water in the vehicle. The water could be condensate trapped in the bottom of the airbox due to the drain hose being plugged (very common on Yaris if the cabin air filter is not installed in the slot behind the glove box). It could also just be moisture trapped in the carpet or seats.

Running the A/C along with the heat should help, but if it is carpet or seat moisture (from an accidental window left open during rain, etc.) running a household dehumidifier inside of the car on a warm (60F+) day will help to suck it out.