That will get you half way there and the ground side of the coax should attach to the vehicle roof near the base of the antenna, which is not practical.
What you can do instead is capacitor couple the ground which can work the same as a direct connection and I do this for mag mount HF antennas. For frequencies much lower than CB I got an 8 1/2 X 11" thin flexible magnet like you find on the back of refrigerator magnets. One side had peel and stick glue and I glued to it an 8 1/2 X 11" thin copper sheet about 5 thousands thick, then soldered a very short braid to the copper with a lug that attaches to my mag mount.
In testing my various HF antennas with just a large 4 magnet mount, they will not match very well and the match changes all over the place when you touch the coax, etc. With the large capacitor coupled ground attached the match is the same as when I use a permanent grounded mount and its very stable.
For a permanent mount antenna on a bracket above the roof you might be able to use a couple of wide strips of conductive tape, copper or aluminum, with one side attached to the ground of the antenna bracket and have a foot or so of the tape stuck to the roof heading away from the mount. A few lengths of tape running in different directions would be better. This will capacitor couple the ground some and can be removed without hurting the paint.
The key with any of my suggestions is having the feedpoint or base of the antenna as close as you can get to the roof.
prcguy
For CB frequencies you could get away with a much smaller magnet and copper sheet but I would consider about 6in square about the smallest I would try.
Make a bracket that attaches to the headache rack that comes down and extends out over the roof of the cab and make the bracket so the base of the antenna is as close to the roof of the cab as possible