If you are unlucky enough to have a truck with a fibreglass cab, then fitting a CB can be a problem.
Obviously you can not use a mag mount, and both a gutter mount and roof mount although they can be fitted will not work because all antenna need metal under them to act as a groundplane.
This is the best thing we have found so far - and it does work.
The mirror mount has a small "antenna" sticking out underneath it, this is a groundplane. This effectively turns the antenna into a "dipole" and it needs no extra earthing or groundplane to work.
It will not work as well as a normal antenna fitted onto a normal metal cab, but it is the best that we have come up with so far.
Although mostly used on trucks, you could obviously use this anywhere that a groundplane is not available.