In my opinion, if you have a GREAT paint job (I don't) you are probably way ahead to scratch (pun intended) the idea of a mag-mount antenna.
For one thing, all you'd have to do is hit one low tree branch or similar problem, and that might just knock the thing off and down the rear/ side of the car.
They don't work all that well, but you CAN get "dual" antennas which replace your automotive car radio antenna. These have a coupler, and are specially built to operate on AM, FM, and CB
I did once have a couple of truck lip mounts that clamp on, but they STILL are capable of marring the paint, and especially if the trunk gap is "tight" vibration/ movement of the trunk lid may result in damage to one side or the other
If you are "handy" and especially if you have a trailer hitch, consider building a removable bracket to attach an antenna coming out from under the bumper. You can buy what is known as a "1/2 wave design" for fiberglass boats which do not need a ground plane.
