Have you ever driven down the road on a rainy day with fogged up car windows and wondered, “How do I defog these quickly?” Then, you look at other cars next to you at a stop light, and their windows are crystal clear. What are you doing wrong? How can you fix it fast?

The most common mistake people make is hitting the re-circulation button on their climate control (heater control panel). When this button is pushed, the blower motor will open a vent door inside the vehicle, causing the blower motor to draw air in from the cabin of the vehicle, hence the term (re-circulation).

When the climate control is in recirculation mode, the blower motor sounds like it’s moving more air because the fan gets louder inside of the car, the fan sounds louder, but it’s only because the recirculation door is open. If you are recalculating the air inside the vehicle and there is moisture inside your car, it will start to accumulate on your windows.
How to Clear Up Fogged Up Car Windows

If you have fogged car windows, here’s the best way to defog them quickly:

Stop recalculating the air in the vehicle and turn on your air conditioner. The air conditioner will remove the moisture inside the vehicle by circulating the air through the cold fins in the evaporator, which will turn the humidity in the air, back into a liquid, and remove it from the inside of your car.
Notes:

If your air conditioning does not work, you will have to turn your blower on full defog and open your windows just a little to allow the moisture to escape.
Do not use recirculation at any time unless your air conditioner is working properly. Using it without AC will only cause problems.
Other Causes

Here are some other common causes of foggy car windows:

If your floors are wet in the vehicle, and there is no good explanation for it—like if you left your window open in a rain storm—you may have a water leak in your vehicle. It’s not uncommon to have water leaks and even new vehicles could leak water into the cabin through a split seam, or a seam that was not sealed properly from the factory. It doesn’t happen often, but it does happen.

If you have had your windshield replaced recently, it may not be sealed properly. This is very common, especially if the person who replaced it did the job in 15 minutes. A good technician will take anywhere from one to two hours to replace a windshield properly.

A leak in your heater core could also be the culprit. If you start your engine and your windows fog up immediately, and you smell something sweet like candy, chances are your heater core is leaking. Engine coolant or antifreeze smells and even tastes sweet, that’s why animals drink it and get very sick or even die. Never leave it out where kids or animals can get to it.

If you notice a colorful fluid (usually green or orange) leaking on the floor from under the dash, that could be a sign of your heater core leaking. Heater core leakage is not that common these days, but some do fail. The cost of replacing one is very expensive on most vehicles—generally you have to remove the dash to get to it.
If You Own a Newer Vehicle

Most newer cars are manufactured with climate control, so the user doesn’t have to decide what buttons to push to defog the windows.
On new cars, when using the windshield defogger, the air conditioning automatically comes on even if you are using the heat. It acts as a dehumidifier, clearing your windows quickly and efficiently, simplifying your driving experience even more so you never have to think about what button to push for the quickest results.

Tip: To increase your visibility in the rain, I highly recommend using Rain-X on all your vehicle windows. Driving at night with it on your windshield improves visibility. Best of all, you don’t even have to use your wipers unless it’s raining very hard. That’s just my experience. It also repels dirt and debris from sticking to the glass, which means less cleaning for you.

Source: https://brandspurng.com/how-to-defog-car-windows-fast/

Good one OP

Using the AC without recirculating, works fastest. But how about the rear shield, asides using the heater?

What's Rain-X?

Tinyemeka:
Using the AC without recirculating, works fastest. But how about the rear shield, asides using the heater?

What's Rain-X?

Some sort of water repellant. It forces water to bead and roll off of the car, often without needing wipers.

Nice tips OPs.

It came at the appropriate time.

Bravo.