Your vehicle is the ground plane for your antenna. This dictates that the placement of your antenna effects the radiated pattern. For the most evenly radiated pattern vehicle center is best... on the roof in the middle. Anything else suffers in some direct regarding the radiated pattern. However you can use that to your advantage. The body tends to pull the radiated pattern a bit. So, mount in the left rear tends to pull the radiated pattern forward and right... a little. It does however fall short in the rear left reflected range.
So for my needs where serious communications is most critical, the ham antenna is vehicle center, the cb antenna is mounted in the right rear. I care about the most range as possible with ham, not cb. If cb was my main concern, and maximum range was required, vehicle center. The difference in range between the two is 50 plus miles with the ham (hit a repeater and it is state wide), 2 miles MAYBE with the cb... as a rough example... other benefits apply as well. I'd love to see a bigger push towards ham vs cb for offroading communications... if for nothing other than the safety aspect of really being able to reach out and touch someone.
For a great CB antenna mount vehicle center look for NMO.