As I understand the 'defogger' for the windscreen consists of blowing hot air up from the bottom of the windscreen. There are claims that rolling the windows a crack speeds up the defogging - how does that work? What can be done when the windows have to be all the way up; e.g. in case of rain, snow or strong winds?
Turning on the air conditioner helps dry out the air. It seems counter-intuitive to have both the heat and the air conditioning running at the same time, but I personally have found that it works far better than rolling down a window or using the heat on the defog setting alone. I've also heard that running the AC in the winter time is good to help keep the seals from drying out.
To be clear, I'm saying to turn the heat all the way to the hottest setting, turn the air deflector to the "defogger" setting so that the hot air blows up along the windshield, and then also turn on the air conditioning.
The only way I know as to each of the hundreds of brands, models and years of cars is to read the owner's manual and to follow instructions. All cars are not sealed but have a designed rate of air exchange even when the A/C is off. If all is working there is no way to improve. If you are having issues with this feature, it is best to have a mechanic check the car. For example, allcars from 15 years ago +- have a cabin air filter. If clogged the volume of air could be less than optimum. To defog the A/C compressor must be in operating order as it takes moisture out. The A/C is very complex and each tech must be certified to repair.