I had made a mount that attached to the head rest posts once upon a time. It was great on the street in my Focus, but the seats in our rigs move so much, it made for a worthless shot, especially off pavement. Right now I have a suction cup mount, and a clip permanently mounted to the dash that the cam can attach to, but I like the shot from the back seat that shows driver inputs and such. It makes for a better sense of the speed you're travelling. The shot from in the windshield always appears much slower on video than it actually is. I think my next mount will be a spring bar (shower curtain rod?) between the two back doors with a motorcycle handlebar mount attached to it.
I also could do the handlebar mount on the cargo rack in the back, but I'd have to zoom the camera a bit to get a better shot through the windshield, and zoomed shots tend to be a bit blurry.