The fastest way to defrog wind screen is..
- point the blower to wind screen.
- turn on the AC and switch the temperature to hot mode.
- keep the blower to your liking.
That way you would still get the warm air and window will get clear in no time. Then you can switch off the AC till wind screen gets foggy again.
If you do not have working AC, you can also try switching to fresh-air mode. The downside of that is if you have a lots of diesel smoke coming from cars in front, you will have to inhale it :P.