My newborn's car seat faces the rear of the car, and I can't see her while driving, so I got a GoPro knockoff, and I can monitor the video on my mobile phone over Wifi. This Thing fits into one of the headrest holes of my 2013 Ford Explorer, and a GP handlebar mount fits snugly over it. (If you're like me, you have more GP mounts than necessary, so devoting one to this noble use won't bother you.)

Printed 100%, PLA and fits with no sanding (not guaranteed :-) If you build one let me know... I'm not sure how standardized headrest mountings are.