So here is a link to a template I made for the GM camera system only.
For the larger square, on the outside, it was just about exactly the size of in between the license plate lamps. The inside square is the area needed to be cutout for the GM camera. The inside square drawn is slightly smaller than you'll need to have the camera fit through. I figured it would be better to start out a little small and then you can trim it and customize it to your liking.
Believe it or not I have a couple of hours into the template. The original was done on a piece of cardboard, which I traced to post here.
The small rectangle is almost exactly the size of each side of the camera bracket. I gave you that template as well. I wanted my holes to be just about perfectly centered, and on both sides, since I am OCD so I made the template.
This is for the GM camera system only. If you don't like my template, then don't use it