If you have a small DV cam like mine's here in the picture... you can just sandwich them between the headrest. I put cloth around it for more cushioning. Sounds are a bit muted tho and when you go over potholes.. you can hear the camera shake. Other than that.. the video is pretty stable

 

and here's a screenshot of the view from the headrest....

 

That's kind of what I did when I made my Borla MPG video. Works fairly well.