The fog comes from water condensing on the windshield or windows.
The problem is that the air inside the car is warmer and/or wetter than outside.
(e.g., https://van.physics.illinois.edu/qa/listing.php?id=1511 )
One of the faster ways is to lower your windows to lower the temperature in the car, and decrease the humidity by bringing in outside air.
This may not be particularly comfortable but it definitely works.
lifehacks.stackexchange.com/users/26/Geoff Hutchison
Since the fog comes from moisture inside the car a simple solution is to dehumidify the car by filling an old sock with kitty litter and placing it inside the car in an inconspicuous place like under a seat.
You might want to use an old pair of socks and double wrap the kitty litter.
Video from Huffington post of the kitty liter trick
But did you know there's a super easy way to beat motor mist?
Simply put some cat litter in a sock, leave it in your car
and you'll be on the road without delay every time.
lifehacks.stackexchange.com/users/117/Jon
If wiping leaves streaks, then there is something on the glass (oils from wiping with a rag that isn't perfectly clean etc)
so it will not get better until properly cleaned.
You need to really give your windscreen a good clean, and then use a product like Rain-X's window defogger (it's a window cleaner with something else that prevents condensation) - and it will last for months (possibly years - not sure) Also, using aircon helps to dry out the air in your car, reducing the problem.
lifehacks.stackexchange.com/users/94/Rory Alsop
Instead of using warm air to defog your windshield, use cold air - it is much faster.
Also, instead of putting on the air conditioner, switch it to windshield mode.
lifehacks.stackexchange.com/users/4223/SAS2507
Prevent the fog by wiping the window/windshield by a cloth soaked in shampoo water or soap water!
This is just a temporary solution but it will be helpful sometimes!
lifehacks.stackexchange.com/users/2386/Dushyanth Shenoy
Put a mains powered fan heater in the car for a few minutes before leaving home.
(Think about safety risks when doing this.)
lifehacks.stackexchange.com/users/1624/Ian Ringrose
If I don't warm up my car in the mornings the windshield gets so foggy that I can't see.  The Heat/AC doesn't seem to help fast enough, and wiping the windshield leaves streaks.  How can I deal with this quickly so I can get back to driving safely?
What is the quickest way to defog car windows?
lifehacks.stackexchange.com
<winter><car><glass><air>
lifehacks.stackexchange.com/questions/85
lifehacks.stackexchange.com/users/75/Bobo
One fast way to defog car windows is to open the windows to lower the temperature and decrease the humidity in the car. You could also consider placing a box of kitty litter in the car to reduce the humidity. Finally, you might want to think about using cold air rather than warm air to defog the windows. There are many possible solutions - you could lower your windows to equalise the temperature, keep an old sock filled with kitty litter in the car, clean the windscreen then apply a defogger, switch the aircon to windscreen mode on cold, wipe the windscreen with a cloth soaked in shampoo or warm the car with a mains-powered fan for a few minutes  before leaving home.
Lower your car windows to equalise the temperature inside and outside the car - not so comfortable but effective. Keep an old sock filled with kitty litter to absorb the moisture. Give the windscreen a good clean and then use a defogger like Rain-X. Switch the aircon to windshield mode and use cold air. Wipe the windscreen with a cloth soaked in shampoo. Put a mains powered fan heater in the car for a few minutes before leaving home.