UP ON TOP, as close to TRUCK center not roof center as possible
1. Most of the transmitted signal leaves the antenna at the base
2. Most CB antennas are ground plane dependant in simple terms the truck body helps "pull" the signal out of the antenna
3. Low mounting looses directional transmit, ie mount on bed rails lose performance transmitting forward, mount on front fenders lose performance transmitting to the rear... .
To maximize performance mount antenna as high as possible, ie on top, make sure you get an antenna capable of being tuned, use a swr meter to tune antenna, make sure to ground radio chassis to body not battery
personal note: I prefer a wire antenna instead of fiberglass, why less noise, mounted on top less flex in truck roof from wind resistance, the whip can be replaced CHEAP as long as base is still good. I use Wilson's (no endorsement, promotion, or sales pitch) just my preference, they can be had for cheap over the web, in stores some tend to be costly.