Originally Posted By: motor_oil_madman
How about the bumper behind the license plate?
Might work but often bumpers are isolated from the vehicle ground due to safety provisions (foam, rubber bushings, etc). But there will be a good ground at your license plate lamp.
A good ground is essential in automotive systems. You can always run a wire from your camera to any convenient point that you know is part of the frame or chassis. Remember in a car current flows from negative (ground) to positive, so the ground is a critical part of any electrical connection.