Stationary installations are fairly straight forward. My
company provides Cellular Repeaters and Towers for the Oil and Gas Industry, and while we have done some vehicle installations, you never get the same range as you would with a full blown repeater/tower setup.
A problem occurs when you require a high gain repeater (75-85 dBm) because of the remote nature of your location and you don't have the necessary antenna isolation between your Outside and Inside antennas. Obviously, on a vehicle this is particularly challenging, unless you field a portable tower of some kind.
I recently cobbled together a system that combines the high gain, directional aspects of tower based repeaters in a mobile package. I've been fairly pleased with the results, but it does require the user to flip up and rotate a directional antenna to find signal in really remote areas. I was curious if anyone else had come up with any other solutions.