However you mount it, you want good metal to metal contact between the outer part of the mount, and the vehicle earth. this means checking the connection to the roll cage, and also the cage to chassis mounts, for electrical conductivity.
Being pedantic, and probably beyond your requirements;
'Earth' and 'Ground Plane' aren't the same thing. whilst creating a connection to earth is relatively easy, a ground plane takes more thought. Ideally, an aerial would be mounted on to a flat metal surface, of large area and uniform shape. A roof isn't bad if you mount the aerial in the middle. If you were to mount the aerial at either end of the roof, the shape of the roof/ground plane would shape the performance of the aerial in different directions. If you're using the roll cage, the above doesn't really apply apart from the fact the earth plane is far from perfect.