Lets get back to the topic...
A Hitch mount Will work, just not as well as it would if there was some metal under the antenna. The link that W9 posted to K0BG`s web site is full of loads of good info.
HV posted to keep the antenna 2 feet away from the hatch or glass makes it worse, read through K0`s web site and it will explain why.
You would be better off bumper mounting than having it extended away from the vehicle at the lowest point of the car.
I have seen large HF antennas on a hitch mount used but that is a totally different animal and if you are only running a single band ( CB) you can get much better results getting the antenna up and having a proper ground plane under it.
Jeff