chocolate teapot, wiremesh watering can, tit on a fish, pick your anaology.
I bought 2 or 3 when the matrix in my old motor went, absoloutly pointless, the best I got, with the best one agains the screen was a hand sized hole to see through and that was after 8miles.
The better option was a fan heater in the car, hooked up to the mains on a timer, that defrosted the car and kept it warm for the length of time it took me to get to work.