[purple gas]

Living in Saskatchewan I know a bit about frosted / fogged windows in cold weather.
If you're not getting outside air or evacuating moist air from the cab and it's cold outside that moisture is going to condense on the inside of the glass.
I used to work in Car rental. In winter people complained they couldn't keep the windows clear. invariably I'd look at the heat controls and the recirculation button would be on.
I'd push the button to select outside air and within a block on the trip driving the customer home, the windows would clear.
Being a highly trained professional if they asked why they had problems with frosty windows but suddenly they were good I wouldn't say " 'casue you're a f...ing idiot".
You got people breathing, often snow from their boots melting, all adding humidity to the air. No matter how high you turn the heat, the glass will be cool, even cold.
Basic science, have something cold and if the air is humid moisture will condense on the cold object.
That works slightly different in southern humid climates. There's different principals involved. If it's winter driving you're talking about, if you can't get outside air into your system you can always crack a window a bit.