When I push this button, current runs through tiny wires in my car's back window, and the resistance to this heats the window up. The head makes water deposited on this window by condensation evaporate, making the window clear for me to see out of. This is legally mandated for my car, as lawmakers think it a reasonable expectation that I be able to see what the hell is going on behind me when I drive.
The windshield, though, or front window, is defogged by blowing hot air from the engine. This is partially because the engine is right below it, and this is a ready source of heat, but mostly because the little wires would be way too prominent at that close a distance. The back window is six feet away from me, the driver. The little black lines are just too tiny to notice at that distance. The windshield, though, is merely inches away. To put little wires here would mean looking a little black lines constantly while I drive.
Let us suppose, though, that I didn't mind the wires, but needed some sort of automated system, so I don't have to bother with the button. I think I would resolve this by materials science.
I would need to produce a material that conducted electricity, but developed greater resistance under low temperatures. In temperate environments, where the window does not fog up, the wires conduct efficiently, and very little heat is produced. However, in colder environments where my own body heat makes a temperature gradient that encourages condensation, the resistance rises considerably, producing heat, and evaporating this fog before it can really get started.
Nah, there's defoggers set up the way they are for a reason.
However, if I were redesigning cars, I would make each of its vents independent. The front vents for blowing air at my face, the lower vents that blow air at my feet, and the vents that blow air at the outer windshield would each have their own dial for temperature and amount of air. As I turn the amount of air dials up, a fan in the vent is given more voltage, making it rotate faster and push more air. As I turn the temperature dial up, more of the air is routed past the radiator.