Some good footage was acquired by Ruut here by a suction cup on the bonnet.
Click here... Thanks Richard. Today we were touring again and experimenting with our GoPro. I now think I'll buy a GoPro Jaws Flex Clamp, easily to attach to e.g. your luggage rack, permitting from a somewhat higher point of view the image from the rear, just seeing the driver / passenger, and their view. Otherwise, it becomes a little to "static".
For the rest, I must say that the suction cup is very steady and reliable on the bonnet, windscreen, etc. But I search for another point of view.