1. if the fog appear on the internal of front windscreem when driving during winter, basically it is due to the temperature outside is lower, causing front and rear windscreen temperature become lower too. When the temperature inside car risen, the air inside the car in contact with front and rear windscreen condense become water droplets which attach on the windscreen, forming the scenario of "fog" appear on the glass. The solution is: Turn on the AC switch and set air flow direction to front windscreen, shut or reduce at least 3 others aircond ven and set the blower motor speed to ideal position. If the air outside the car is clean and dry, then can switch off the air recircullation switch to draw in the air outside or vice versa. Besides, set the temperature to an ideal position to make the cabin temperature feel comfortable. If you have glass anti-fog spray/gel, may put it to use. Usually it can be purchased from the car accessories or car wash shop and it might not cost must but the effect is impressive. Lastly, if you do not have the anti-fog spray for car, may use the glass cleaner for household. Use it to dampened a cloth and wipe on the glass inner surface, then wipe the glass to complete dry.
2. If the above situation did not describe accurately about the problem you are facing, kindly proceed to workshop for diagnose for the sake of driving safety.