You can buy some of that Rain-X anti fog stuff, you rub it into your windows glass, then wipe it clean. No more fog.

Also fog is usually caused by moisture in the air, but when its cold, air can't hold moisture. As air heats up, it can hold more and more moisture. This is why blowing cold air will usually alleviate the problem.

If its wet and cold out, and you put on the heat, you might get the fogging effect. The moisture in your clothes will evaporate and get trapped in the hot air in your car. If it's just cold, theres really no moisture in the air, so you shouldn't have a fog problem, even with the heat on full blast.

I have a moisture problem in my basement, and in the winter its fine. In the summer I need the dehumidifier on high. All a dehumidifier does is chill the air so moisture condenses and falls in the bucket, then reheats it to room temp and blows it back out without the moisture.

I know none of this is a real solution to your problem but maybe it can help you figure out whats causing it. I'd say if you don't know the source of moisture, try blowing the AC until the fog disappears, then put on the heat. If it keeps reoccuring, it could be something with your car. Water in the vents? I dont know. But theres probably a moisture source somewhere.