I thought about doing this once. There are antennas that arent the giant white whips. I thought about trying to use the rear window somehow, like mount the antennas on the inside of the car. My understanding of how they work is that they do have to be straight up and down, and the more metal between the antenna and the receiving radio the better the signal will be. Another option you might want to look at is a removable magnetic based antenna. We use them on the work trucks because when they are in transport the antenna will catch on the over passes