You didn't mention if the antenna was a full-sized quarter wave-length or a shorter antenna with a loading coil. This makes a difference for two different reasons. I will handle the first reason immediately -- no full sized quarter-wave 11-meter (CB in US) antenna should be mounted with a magnetic mount -- although the "wrongness" of this type of mount suggests it is likely a shorter antenna with a loading coil.
Assuming your tool box is mounted just behind the cab of the truck then I believe that your antenna will be below (or mostly below) much of the cab which is a lot of conductive metal. This is not good for having the best performance from your antenna. In fact, you can experiment with this easily (especially for magnetic mount) is to try doing tests (signal strength etc.) with the antenna temporarily placed at different locations on your truck.
Most of the coil loaded CB antennas I have seen have a center mounted loading coil. This means that the higher current part of the antenna which produces your strongest radiation, is between the base of the antenna (at the feed point) and the center loaded coil. This part of the antenna should be clear from as much of the metal of the truck as possible (metal below the antenna is OK).