If the antenna is mounted to the bumper, you can go with a conventional antenna. If you really want to mount it to the fiberglass body, you must use a no-groundplane mount. But be aware that no-groundplane antennas do not perform nearly as well as a conventionally mounted antenna does. A regular antenna that is mounted to the frame, bumper, or steel body makes use of the tub/chassis as part of the antenna which is called the groundplane. It's like doubling the size of the antenna. A non-groundplane antenna has to make do without all that so it doesn't perform nearly as well.
Mounting the Antenna in the rear of the vehicle to the bumper or on the tire carrier is the best over-all location.
You won't gain anything by mounting on the top of the vehicle except radiation pattern. If you get the antenna to work correctly on the top of the fiberglass top you will have a radiation pattern that is more circular, on the rear of the vehicle the pattern toward the front of the vehicle, which is usually where you want it.
Save a lot of headaches and mount it on the tire carrier...