Late to the conversation on this one but I have run antennas on trucks in some pretty weird places and they all performed well. The grounding, coax and antenna are all important including the mounting location. I ran a Dodge Dakota with a predator antenna in the rear bed corner near the tailgate with great results. I was running bigger power back then and actually had used heliax coax for the install and mounted all my amps on the inside back cab behind the jump seats. The setup was slightly directional but not as bad as you'd think. I also ran ground straps from the antenna mount to both sides of the frame.
A number of years before that I ran a Ford Explorer with a 102" whip - the mount was a thick piece of flat steel I had connected to the frame and ran straight out the back of the vehicle towards the left back corner so the antenna wouldn't interfere with opening the back hatch. This kept the 102" far enough away from the cab as well so no issues with reflect. I only ran about 400 watts with this setup and it worked really well but was somewhat directional. I used to be able to jump on the bowl and make contacts from Hawaii to Cali no problem.
I also have mounted a 102" directly into the rear steel bumper of a 1979 Chevy silverado and that setup worked really well but again was somewhat directional.
If you want the most omni-directional setup than a magnet mount or thru hole in the top of the cab is best, but if you want to run a long predator or 102" and don't want to hit everything you can definitely mount it to the bed, frame, or bumper off a back corner with pretty good results. Any bed mounted antenna will need some good grounding through, make sure to go straight to the frame.