There are two different types of ground as far as antennas are concerned.  There is DC ground (a physical metal-to-metal connection), and there is RF ground (Radio Frequency), which is a coupling effect instead of a physical connection.  Both are important, but the RF ground is essential.  DC ground is not necessarily needed as long as the RF ground is effective. For example a magnet mount with a protective coating on the bottom is not DC grounded, especially when attached to a painted surface, but the magnet being so close to the metal, forms a fairly good capacitive coupling to the ground plane (the roof). However in this case the roof is at ground potential, if it were not, it would be difficult for the RF to couple to it.

When you mount an antenna on a rack, or load bar, or any accessory like a brush guard or tire carrier, you have neither a good RF ground or DC ground, unless the accessory is well grounded to the body, or you physically ground the accessory yourself somehow.  However, even though the accessory is DC grounded, you do not necessarily have a good coupling to RF ground. The radio waves need to be able to be coupled to ground, but they can get confused when a rack is introduced into the circuit. Because the rack itself does not have enough mass to be an effective ground plane, and it is too far from the roof to be effectively coupled, the radio waves are uncertain what to do with the rack, and how to get coupled to the real ground plane... the roof.  At this point, some very strange things can happen.  Since the rack is too small to be the ground plane, it may try to become part of the antenna, and really cause problems, usually associated with interference of some kind.

Many times a rack mounted antenna will cause the CB transmission to come across the FM stereo loud and clear, while at the same time, other CBs will not be  able to pick up the signal at any distance.  In this case all the transmitted power is being wasted trying to couple to the ground plane, and splattering everywhere but in the right direction.  Adding a ground wire to this rack mount could make things even worse... it too could become part for the radiating element, ...then you would have three antennas!

It is impossible to predict exactly what problems will be encountered, so it is accepted practice by professional radio installers to avoid using this type of location. It is considered too unpredictable, even though there may be no noticeable side affects at all.  Every situation is different and will have different results because the CB's frequency range is highly susceptible to interference from anything and everything.

If you are unable (or unwilling) to use a better location,  the best means of grounding a rack is through the mounting hardware (like gutter clamps), with a metal-to-metal contact. The next best way is with a wide flat braided ground strap, do not use a wire.  A rack should be grounded at all four corners, a light bar at both ends, and a tire carrier should be grounded at each hinge (don't expect the hinge to provide the ground).  Always be sure the antenna has a good DC ground to the bar (no paint in between). Though it may seem to work okay, you must understand that any mount of this type will compromise your CB's performance.