Kent,
Never tried it, but I doubt a preamp would be worth the expense if mounted at the rig in a mobil set-up. I assume the feedline is only 10' or so. The preamp helps most when the feedline is lengthy, especially if cheap coax is used.
If you want to try a switching preamp, the ARR SP432VDG is a good one, see http://www.advancedreceiver.com/page7.html
For about the same price, you could buy an Arrow handheld yagi antenna, which would substantially improve both reception and transmission. See http://www.arrowantennas.com/146-437.html , although if the rig is fixed inside your car if might not be convenient to hold the Arrow, unless your mike cord is long enough so you can stand outside the car.
If you are limited to operating from the car, try driving to a hilltop or somewhere with a clear view from horizon to horizon.
73, Bill NZ5N
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