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ichitaka05 I have a sorta novel idea. Wyze cam. They are powered using usb. Pretty dang good night vision (I use it at night to check on haru when he's sleeping and it's dark). It should be more than sufficient for a backseat. I would assume a car charger should be sufficient.
Although only thing is it may require wifi router... check if there's a way to avoid that. Or maybe a similar camera that you can connect to directly with a phone. I think I've heard of similar cameras for hunting perches and I doubt those are connected to a wifi router.