Trust me it takes a lot of electrical power to make a small heater put out he heat you want, any these little mickey mouse heaters dont have what it takes with only 12 volts. I had one a few years ago , what a joke!
Fact is, creating heat from an electrical source requires lots of power. Not that it's a bad idea or anything, it just seems pointless when you have 200F+ coolant flowing in the car just waiting to be cooled. Is there something wrong with the heat in your car?
The only time electrical heat in a car makes sense is if you'll be sitting in it for long periods without the engine idling.
The defrost option in your car will dehumidify the air that you blow onto windshield. De-humidification is done with the automatic use of the A/C compressor.
Blowing only slightly heated (but still humid) air on the cold windshield (cooled from outside air) will not help as you think.