Never owned a vette and your assumption that there no ground plane would likely be wrong. This is the laws of physics, called radio wave propagation. A mast antenna requires a ground plane. Cudamark think about it two professionals have said a ground plane is required, if I were to guess, just a guess, there's steel embedded in the fiberglass. I worked for 30 yrs developing Rf designs Transmitters 5 to 10,000,000 watts, receivers 100 khz to 10 Ghz, and antennas 100 khz to 5 Ghz.