And, once that magnet foil capacitor is mounted on the car and the RF couples to the car sheet metal, the body of the car is the RF "ground" -- or more exactly, the "counterpoise" for the antenna.
Good ones are made that way, but I've seen many mag mounts that unfortunately aren't.
A local ham friend brought over a "Firestick" CB antenna mag-mount with a very powerful magnet and coax connected to it. It worked terribly. I measured continuity between the shell of the coax connector and the base of the mag mount, and there wasn't any at all -- complete open circuit.
So, I took the base apart. Inside, the coax outer conductor was bolted directly to the magnet, but there was no foil or anything anywhere (factory stock), and between the magnet and the outside world was a thin layer of plastic (anti-scratch), but no foil in between.
The magnet was 100% non-conductive (like millions of Ohms) and the magnet itself evidently was intended to be the "capacitor." It didn't do a good job of that.
Replacing the mount with one from Larsen took care of the whole problem.