There are a number of mounts that normally attach to the edge of the trunk lid. Comet is one of those manufacturers.
You need to know what kind of mount your antenna uses. If it is a PL259 coax connector, then there is a Comet mount that will attach to the edge of the tailgate. It has to planes of movement, so you can adjust it so the antenna is pointed up no matter what angle the tailgate is when it is closed.
MFJ is a supplier of all kinds of radio equipment. Their website is
www.mfjenterprises.com There are two Comet mounts. The MFJ catalog numbers are:
MFJ-345S - $34.95
MFJ-348S - $36.95
The 348S has a thumb screw that may be easier to adjust.
Both come with coax that you can route to your CB radio.
I'd suggest that you go to the MFJ websites and search for those model numbers.
You might also search e-bay, etc. for comparison pricing.
Hope this helps.
W8RLM - Bob