Hmmm...my (class C) has the radio antenna in the factory position on the fender, which I always thought was a bit silly because it can't extend all the way due to the cabover and wondered why they hadn't bothered to relocate it. I think I may be reevaluating my opinion.
Many cars these days use either a stubby or "shark-fin" style antenna on the roof, some of which have a built-in antenna amplifier. Maybe something like that would be effective enough and unlikely to cause roof damage. The trick would be finding a car that uses one and has a (locally) flat roof...or one close enough to being flat to work acceptably.