Why Do My Mirrors Fog Up and Freeze?
In order to stop your mirrors fogging and/or freezing, you need to understand why it happens in the first place. The air is full of water vapour; this vapour turns to liquid as it hits the glass of your mirror and then sticks to it. The water molecules also stick to one-another. When the temperature drops below zero Celsius, the stuck water will freeze on the surface of the glass.
To keep the mirrors from collecting this water, you need to make their surface less sticky, and take advantage of how the water molecules sick together. To keep the water from turning to ice, you need to make sure the mirror's surface doesn't get too cold.
Using Wax to Keep Water Off
Wax is "hydrophobic" which means it repels water. Coating your rear view side mirrors with a thin layer of wax will keep the water molecules off. Make sure to invest in a wax that has been specifically designed for car mirrors, such as RainX; using a different kind of wax may not achieve the desired effect and may even negatively impact your rear and side visibility.
Find a soft dusting cloth and place a small amount of wax onto it. Do not use an abrasive cloth as it may scratch the glass. Rub the wax thinly over the entire surface of each mirror. Finally, buff the mirrors until they are shiny, and the reflection does not appear too distorted.
The layer of wax not only reduces the stickiness of the mirror, but also uses the water's own stickiness to clear it away. You may not know that the apparently smooth surface of the mirror is actually covered in tiny indentations that the water can sit in. Wax fills these indentations, and causes the water to run into one big stream so that it falls down and off the surface.