Antenna mounting location
The problem you run into with mounting an antenna on a offroad vehicle is the compromise between protecting the antenna and providing a good location for the antenna to work well.
Most vehicle antennas require a counterpoise, or ground, to work well. For that the best place is usually right in the middle of your steel roof. For protection, not to mention not wanting to drill a hole in the middle of your roof, that can be a poor location.
Mounting on a rear bumper or spare tire carrier, while protecting the antenna from tree limbs, can have a very detrimental effect on propogation, often reducing the gain of the antenna by several dB.
I use a Diamond NR770B dual band (2m/70cm) antenna on my 100 series. It's currently mounted on a fender lip mount and grounded to the body. I get excellent signal reports from most listeners whether working repeaters or simplex. Formerly, I ran the same antenna mounted on an ARB combo bar on a 2000 Landrover Discovery. It also worked great in nearly every direction.
An interesting aside; I used to frequently chase down aircraft ELTs using my ICOM 2720 radio and the setup on the Landrover. The bumper mount gave just enough directivity to the antenna that I would get about on S-unit difference depending on whether the ELT was in front of me or behind me. Since most accidental activations took place at the local airport, I could triangulate using the truck and my amateur radio tuned to 121.5 Mhz.without having to use more complex DFing gear.
I haven't tried it with the Landcruiser since we got some cool new DF gear that is much easier to use.
Doug W8PM