To prevent your car's windows/mirrors from icing up... treat the windows before you go to bed each night. The window treatment can be made up of 3 parts vinegar to 1 part water and placed in a spray bottle... [and] will prevent ice from forming.From other posts in the thread, it sounds like rubbing alcohol can be substituted for vinegar, and the solution can be applied either before or after the ice has formed.
From Lifehacker