audi... that explains it...
alot of new cars have antennas that ideally can only be used with the factory radio, because of special amplifaction circuitry, that car for sure is one. now for a while companies made little interfaces that you connect to the antenna, then to the power antenna lead of the aftermarket radio, to enable the amplifers in the on glass or stubby antennas
WV was the 1st company to use this technolgy
i would not use a FM booster, they cause issues too