tl;dr of my post: turn on AC and blast hot air at the windshield. if that doesn't work in the winter, open your windows. if it doesn't work in the summer, it's

tl;dr of my post: turn on AC and blast hot air at the windshield. if that doesn't work in the winter, open your windows. if it doesn't work in the summer, it's because the fog is on the outside of the windshield; use your wipers.