Mounting location is best where the most of the antenna is above the highest point on the vehicle.

Choice of location depends on vehicle. Roof mount is best if you have a metal roof. Using a piece of sheet metal on a fiberglass roof does provide a functional ground plane.

Tire racks provide a good mounting location as it does place the antenna high and the metal provides a good ground plane.

Windshield is another good location.

Bumpers are not optimum for a 2-meter antenna as that places the majority of the antenna low and the metal of the vehicle does act a shield for the radio waves.

For a vehicle (mobile installation), a whip style antenna is best. It is easy to mount and will withstand wind and contact with brush and tree limbs.

Other antenna styles are superior in performance but not as durable in mobile installations.

Check http://www.elkantennas.com/2m4405element.htm

That is a very good portable antenna that can be used for mobile installations. A friend does use one on his trail excursions.

However, it is susceptible to damage from brush and tree limbs.