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The vehicle would function as a ground plane to return induced currents to the feed at the base of the antenna. At BCB wavelengths the feedline ground to the chassis would probably serve this purpose and customary practice is to ground the feedline to the chassis at the base of the antenna. Given the large dimensions and loading coils used for HF mag. mounts may be an interesting idea but hardly practical.
As we go to higher frequencies, especially VHF, the feedline becomes a significant portion of the wavelength and the best and most practical approach is to provide a RF ground at the antenna base either by direct ohmic connection or capacitive coupling. For example, my 2 meter (144 MHz) mag. mount has a base 2 inches in diameter and that is sufficient.