If you're expecting ice to form on your car's windshield overnight, then you might be able to prevent it from icing up using vinegar. Mix water and vinegar and wipe it on your car's windshield the night before, and it'll help prevent icing and causing you to have to scrape your windshield in the morning. This works because the acid in the vinegar lowers the freezing point of water.
However, it won't help if your window is already frozen over, so it needs to be put on the night before. You can use a spray bottle for easy access everytime you think temperatures are going to plummet.