Forgive me, but this is taking me way back. Most here in the US don't use CB anymore. Cell phones rule, however, I miss the old days chatting with fellow travelers on long trips. (Never see a cell # on the back of an RV)
Why not mount it on the roof? It'll be higher and take advantage of the ground plane. Even easier would be a magnet mount dead center on the roof with the coax clipped to the roof rack and then leading into the rear window or even closed in the door's top.
'06 Commander Limited, Black, Hemi, Nav; '04 Cadillac SRX AWD, Blue, N*, UltraView, NAV; '00 Regal 29' Commodore