When overnight temperatures drop below freezing, it becomes the norm to spend ten or 15 minutes each morning to scrape snow and ice from the outside of your car. But when the ice is on the inside of the windshield it's a different story. Ice scrapers and deicer both work just as effectively on the inside of the car as they do on the outside, but melt the interior ice and you're often left with a miniature swimming pool on the dashboard. The key is to deal with the moisture before it condenses on the windows and freezes.
Locate the source of the water in the vehicle. There may be leaks, perhaps wet or damp items are being transported in the vehicle, maybe some of the upholstery has become damp, or perhaps a change in the weather means passengers are carrying more moisture than usual.
Remove damp items from the vehicle and fix any leaks you have identified. This prevents more water from being introduced into the vehicle in future.
Open the car windows and turn on the air conditioning to a high power level, at a warm setting. Switch on the window heaters, if available. Do this for at least 10 or 15 minutes to dry the car out. If you have discovered an excessively damp area, for example if a seat or carpet has become very wet, apply heat and ventilation to it directly with a hairdryer or other heater.
Place a large bag of silica gel or other desiccant under a seat or in the boot of the car, if the problem persists. This soaks up moisture from the air and will help prevent condensation. Cat litter has a similar effect.
Cover affected windows with a blanket or cardboard overnight. This is a relatively extreme step and usually isn't necessary if the interior moisture problem has been dealt with.
Purpose-made spray-on products do exist for interior window ice prevention. These products are designed to be sprayed onto the window the night before, and are effective, but only treat the symptoms and not the cause.
If drying or airing your vehicle in the garage by running the car with its air conditioner on, make sure the garage doors are open and do not stay in the garage while the car is running. Ideally, do not run the car in the garage at all, as it poses a risk of carbon monoxide poisoning to humans and animals when a car idles in a confined space.