The above photo is probably the worst place you can mount any antenna. The base is not above metal work and it's to one side which will react with the operation of the antenna no matter what frequecy you operate on.
The ideal position is in the middle of the roof as it provides the best "Ground Plane" for 360 degree signal. If you mount any antenna where the amount of metal is missing from one side. like at the top of a Bull Bar in front of the Bonnet or on the side of your Vehicle Roof then you scew the signal in one direction and reduce it from the other.
If you mount an antenna to one side then you need to create the artificial ground plane, for UHF/VHF you will do better with one of these....
http://www.ebay.com.au/itm/291130643550 ... 1439.l2649 Antennas that state "ground plane independant" is just a sales pitch. When you use very expensive equipment to plot the radiation pattern then you will see what is going wrong. How do I know...???? I have some very expensive equipment, friends that have more and work in the communications business, have done for many years....called Radio Engineers.
I have HF, VHF & UHF Antennas mounted in positions that are not ideal but the best I am going to do as I'm not drilling holes in a roof. The above antenna base I use for VHF/UHF Amateur Radio Communications. Sorry, I don't play with CB much but the principles still apply except you are very limited with the power you can use.
So, if you don't want to drill holes. Mount an Antenna in the best place then it would be on Roof Racks with a nice metal earth below and in the middle of your roof. Make sure all earthing is bonded together.