It would not be mounted on the bumper.
My thinking is that since the first 1/3 of an antenna does the most
radiating, get it as far away from any metal (broadside) that you can.
It would be mounted in a stake-hole (one of the 4 square holes in each
corner of the truck bed.
Joe

A 1/8wl antenna mounted in the center of your roof will probably work
better
than the 1/4 lambda antenna on the bumper.