Sleeping overnight in your car, air? Opening one window slightly? Air it out?

I sleep reguarly overnight in my car. Over the winter, it's really cold, so I keep my windows up, and I've been fine, but I'm worried about not having enough oxygen and dying or being hospitalized. I'm a fisherman and when my bait doesn't have holes,either minnows/minnow bucket or a worm container, the bait dies from a lack of oxygen, just like the time I shut the lid on a small cooler I put minnows in, so they need air.
Anyways, my concern is I'll end up like my bait and die if I don't crack open any of the windows, so should I crack a window open slightly even if it's super cold outside? Over the summer/hot weather, it's really stuffy, so I crack a rear window open just a little bit to let the hot air out and so I can get some oxygen, but even if you close all the windows over the winter, will you be adequately oxygenated? Do you need to crack a window open? If not, WHERE is the air coming from? The vents from where the AC/heat/air comes from? Are those open even if your car is turned off? You know how you can close your vent by pressing that button if there's a smell outside? If you press that button and close it, will you be screwed, or still ok? Don't press it and you're fine? Does air also get in through the cracks of the door? I just want to make sure I'm safe.
Added (1). Anyways, I sleep overnight in a car fairly often. I do have a house, but I rarely go home since it's far from work/school, and I'm upside down on my mortgage so I dont want to sell it, but I admit, I guess I'm living a "homeless" life, though I'm middle class.

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