I don't know if anyone has posted this or if everyone already knows or not, but here it is anyways. I've always wanted a camera mount, but stuff isn't cheap so one day this idea just popped into my head.. and yeah. I would say it should work on all cars, but not all cars have an adjustable head-rest.

On the passenger seat, one is able to raise or lower the head cushion. In the space between the cushion and the seat-backing, one can fit a certain type of camera into the space.
I own a Sony Cyber-shot HX100v, it's a bit too big but that's sort of a good thing because it fits a little tight meaning it won't slip off or anything when I take corners, and the camera won't pan. If you're having trouble with it being stable and panning, try putting something leather around the camera and squeeze it into the space, or if it's small then have the head-cushion holding it down, similar to a vice-grip tool.

Tips:
If anyone does it, it's best if you ride alone or keep your passengers in the rear seats, and to keep the seat in the most up-straight position depending on what you'd like to be recorded. It doesn't have to be recording what's in front of you, you can put the camera backwards to view your rear or at an angle to capture your driving or the window. Your seat is basically your leveler/adjuster
Seat closer to the dash with seat straight up = less dash view and more road view.
Seat straight-backed = you see the hood and road more.

Video quality:
It's not bad for $0, the only one thing that bugs me the most and is also a common problem with camera mounts... vibration/shake/etc. I'm not sure if it's my camera's lens bouncing around in the housing or the housing vibrating with the seat's plastic, but I'll try to find out.

 

Looks good, if only we were able to have a higher angle so we can see the road more. But pretty good for being free lol.