The lack of a flat metal ground plane will have a 6 to 9 dB loss (towards direct without ground plane) over a roof top base loaded CB antenna. FR my experience with an antenna mounted 6" from rear of roof on my crew cab compared to center mount.
Fire Stick no ground plane CB antenna.
FGX - Firestik No Ground Plane CB Antenna
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I've seen antennas mounted on the spare tire bracket and on front bumper.
GMRS Antenna Installation for GMRS UHF antennas, but will give you ideas.