As far as liftgate mounting, make sure yours is metal first. A lot of them are fiberglass so antenna wouldn't be able to ground without adding a jumper.

Just thought I 'd mention it.

As far as what antenna to get, don't skimp especially when using a cheap CB like the Cobra 19. As far as type, I'd go with a good magnet-mount(Wilson or K40) and set it centered on top from side to side and about 6-12 inches back from top of windshield for optimum ground plane. Otherwise, get a Whip(the 1/4 wave 10' if don't mind loosing some range when have to tie it down for clearance) or coil and mount to rear bumper or rear side(if don't mind drilling) as others have mentioned . Just try, if possible, to make sure whip doesn't extend past 13'6" from ground to tip since that is the federal guideline for minimum road height clearance for overpasses and such.

Good luck!!