When our windshields fog up in the winter, it's because moisture from inside the car condenses on the glass and makes it very difficult to see. Water vapor coming in from an open window -- or even from your own breathing -- can fog up a window. Defrosters solve this problem by blowing warm, dry air over the glass. If you're sure your defroster unit is functioning properly but there's still a problem with too much fogging, have your car checked for air leaks around the doors and windows bringing in extra moisture.
It's also important to stay warm and comfortable while driving, since shivering makes it difficult to steer or pay attention to the road. If your heater isn't working, you may have a faulty heater coil. Although heater coils are expensive to replace, it will be worth it during cold winter mornings if you don't want to freeze behind the wheel.