Yeah, do this. It doesn’t intuitively make sense, but your car’s climate system is not like the one at your home; you can run them both at the same time and not create a universe-destroying paradox ;). The cold air goes through the AC condenser which takes the moisture out, then it goes through the heater core and warms up, the end result is your vents are blowing warm, dry air.
The blending controls in your GM are activated by small vacuum lines behind the control. If it was just your front window not defogging I would ask you to check that the air blows toward the windshield when on defrost, then blows to the floor when you change the settings to blow in that direction. Probably should do that anyway to be sure you don’t have a problem with the control. It is blowing correctly for the setting?
Since your side and rear windows are also heavily fogged it is probably not the control. You have water inside your cabin from somewhere.
Pull out the floor mats and feel around. How long has this been going on? If it is recent it could be weather related. Ice freezing can push out the seal on your window creating a leak channel. Check your trunk too. There are usually drain holes on the body, toward the rear, where water that enters from the deck lid seams drains out. These can get plugged by pine needles or other stuff causing the water to overflow into the trunk rather than outside.
A heater core leak will usually, but not always, leak onto the passenger side of the floor, because that is the side where the coolant hoses enter the cabin. Too much stuff going on in the drivers side to route the hoses there, so they are on the passenger side. As mentioned, a coolant leak will smell sort of sweet.
You have water inside, find it and look for the source
I haven’t tried it my self yet, but apparently one miracle cure for car windows fogging/freezing on the inside during winther is a sock filled with cat sand placed for example under the seat. It apparently sucks out the moisture of the air quite effectively. Could be worth a try?
Just make sure the AC is on. I thought that most modern cars automatically turned on the AC compressor when you selected Defrost. It’s very possible that your car’s AC system is not functioning properly. Does it cool the interior in the warmer months? If not, you need a recharge.
You could try running a dehumidifier inside the car and see if that helps. Unlike the other methods mentioned you can see the amount of water removed. But that seems more of a diagnosis step then a curative one, but it could be a starting point in figuring out why
Turning on the A/C in the winter is going to reduce the defrost air flow and A/C unit will NOT function below 60 deg F. (Ambient outside air)
I plow and get a lot of snow back onto the hood and i can feel snow/moisture in my face when this happens because of outside air, that is why as stated above the intake area needs to be clean.
One thing i do a lot is to crack open a window to let moisture out. I also have a small hand scrapper close by and when i get heavy inside frost i scrape it clear.
My “MOST” difficult fog/frost issues is in the bus i drive when its full of yapping teenagers on a cold day and the colder it is the more issues.
I will question if there is a car with A/C that isn’t running A/C in defrost mode? (even though the compressor isn’t engaged below 60 F Chart Here Below 65-60 deg F the pressure switch will prevent compressor clutch engagement.
In short, you need fresh air, and with heavy moisture an escape route (window cracked) good air flow(Do you have a cabin air filter)?
Jeez guys, when you turn on your front defroster on a modern car, what does it default to? That’s right, non-recirculated, outside air. Leave it on that setting for this issue. Unless of course you actually have water in your car. Even then, it’s likely the best option.
OP, you aren’t showering just before driving are you? Early this winter a friend and I both discovered that wet hair is enough to fog up the windshields our new cars.