Fog on wind-screen

Its very simple. During the winter season, the inside temperature is higher than the outside temperature (due to human body temperature plus heating to stay warm). Plus during rainy reason the air is also humid thus when the hot & humid air inside the car comes in contact with cooler temperatures thus results in small water droplets commonly known as "Fog".

Now the AC comes into action. AC's air is dry (as compared to heater's air) and it absorb the humidity and thus the fog. I hope it clears your query. Moreover if you adjust your heating temperature to Max and run the AC you would notice that your AC is running for a very short intervals not like in summer and thus would not be heavy on fuel. You can also use fresh air to remove fog(with heating, AC) but cannot save yourself from diesel smell coming from outside.

its a freak problem... the fog.....never seems to disappear completely from the wind screen and side windows............sometimes it becomes really hard to see through.........heater removes the fog...but then..the inside tepmertaure of the car becomes toooo much............ the simplest soloution i think is to drive the car with window lowered to a certtain extent..but then..u have to deal with dirt and noise from the outside.....so guys...seems like no easy solution for this freak problem !!!!!

windows rolled up and AC turned on solves the problem always (atleast for me). In winter, move temperature gauge to hotter degree to safe yourself from freezing like KULFEE.

use the internal fan blower with ventilation set on fresh air. this is the best it can get. some cars have bad performanced but some are good go dealing with this problem

Neither East nor West
A/C is the best

In summers use A/C + Hot temperature and in winters use A/C + Cold temperature combination and offcourse the vent to the wind shield position.

Other methods as mentioned by @Spectra and @asd are also workable but that needs the car to be moving so that fresh air could enter the cabin!