There are a few schools of thought on antenna mounting on cars; one option used by several organisations, mostly, it has to be said, in the USA, is the NMO (
New
MOtorola) mounting system.
Yes, it tends to require a new hole be drilled into the car body.
But that's where it tends to stop, as the seal tends to be more waterproof than other mountings, and it uses a somewhat different thread than the SO239/PL259 (which itself is not exactly a waterproof itself) system to screw antennas into it.
The problem is that there are no dedicated Amateur or (as far as I know) CB antennas made for this mounting system, it being primarily a "professional" system (note the pompous quote marks there) so we're right back to mag mounts, gutter mounts, corner mounts, boot lip mounts, and similar.
Some get around the 'leaky exit' issue like I did, and drill a hole somewhere not immediately prone to leakage issues, and seal it as best they can.
Some try the capacitative 'through the glass' method which is lossy as ****, by the way, and results in lower received levels of signal to your receiver, and a higher chance of VSWR-related transmitter issues (it's a complex subject even without the maths, and I'm not going to inflict that crap on you all!).
Still fewer draw out the antenna cable through an existing hole (there aren't too many of those in modern cars, aside from the doors, boot, and bonnet), generally through the bulkhead between the cabin and the engine, and route the cable under the vehicle to a point where it can be brought 'up and over' to the antenna, which requires a LONG cable, it has to be said.
And finally, the majority of people trust to luck, the tooth fairy, et al, and take the cable through a door/boot/bonnet and hope the door seal doesn't get too messed up along the way.
At the end of the day, choosing your preferred way of getting the signal to the antenna is down to you, your faith in WD-40, a mop, sponge, bucket, and luck.
Normally, I'd go with an easy solution, as they tend to be easier to fix if (when) they go wrong. That said, I went with the drilled hole approach this time - your mileage might vary!
Have fun!