If you are going to mount your standard, ground plane reliant CB antenna to a spare tire carrier, luggage rack, or other vehicle accessory, including many mirrors, you need to pay special attention to grounding.
Most add on accessories have some type of bushing or insert that is either meant to act as a bushing or as part of the fastening system itself. Luggage and ski racks usually have an expanding rubber bushing that holds the device in place. This is also the case with many side view mirror installations. The bushings are very effective insulators and can cause SWR and performance problems when using an antenna that requires grounded mounts.
On Jeeps and other SUV's with the swing-away spare tire racks the problem is almost the same. Most factory swing-away racks have nylon bushings to facilitate a smooth, noiseless motion. Like rubber, nylon is an excellent insulator. Also, many aftermarket racks have brass and/or ball bearing joints and you cannot count on these to supply and adequate ground at all times … sometimes it is good and other times it is poor or non-existent.
The ground problem should not discourage you from using one of these convenient mounting locations. All you need to do is to run a short (as short as possible) ground wire from the mount to a place on the vehicle that has a good chassis ground. Usually on the swing-away tire racks it is necessary to run the coax down one of the support bars to allow it to be routed to the radio. In those cases, connect a 12ga or heavier black wire to the mount and run it down the support bar with the coax. Ground the end of the wire just inside the hatch or to the taillight bezel.
Ground plane reliant antennas need a good chassis ground at the mount. You cannot use the ground path supplied via the coax cable. As a matter of fact, when checking to make sure your mount is grounded, remove the coax from the radio to eliminate possible errors.