The GoPro and the Contour Roam are popular options for this kind of thing, both have numerous attachment options available, including numerous ways to attach one to a vehicle (even on the outside; lots of car demos use them, as do TV shows). I own a Contour as I prefer its 'bullet' shape given my needs when I bought it (I own the headband attachment) and its simple on/off slider switch (not sure if the newer GoPros have gotten around to implementing this same idea, but the older ones had clunky interfaces; granted, if you're just using it as a dash camera and not for other stuff, 'instant on' video capture may not be a requirement for you).