I have had bad luck with magnet mounts in the past with CB antennas. They either don't stay put or scratch the paint.
Regardless of the type of mount I go with, I have to be careful running the cable inside since there are too many pinch points at the rear lift gate. I'll figure something out. Plus I haven't decided if I am going to run a GMRS radio, 2m/70CM radio or both in the vehicle.
I also have to keep in mind of overall height since my garage door opening is only 7 feet.