I would like Mike's further information on the grounded 5/8th wavelength aerial he's talking about my understanding was always what Nigel just said, the aerial is not grounded it's the antenna input, but the car is used as the ground plane, for the frequency ranges common AM and FM transmition encounters ground transmission is the primary mode, with FM starting to lean towards freespace transmission.
All modern high frequency antennas (basically 800mhz and up) rely on freespace primarily because there is no ground transmition at those frequencies. 2.4ghz is almost completly freespace, although bluetooth and wifi can take advantage of nearfield effects within short ranges, the earth itself will eat the ground signal.
You have to understand something, radiating electro magnetic fields are their own complete energy source in freespace transmition. ground and nearfield effects not withstanding, basically the objects are NOT connected, they're just using the earth as a whole as a transmition media in the case of ground transmition in AM and low FM frequencies (couple hundred Mhz max) the earth itself becomes the equivalent of the shield in a transmission line. Nearfield basically means magnetic coupling, nearfield is practical around 1mhz or so more commonly bellow. Gigahertz and microwave require waveguides or line of site freespace transmition and require not local ground plane although they are effected by objects near them the signal itself only requires vacant space to exist. The Electric and magnetic fields kind of tumble through freespace alternating phase along the way (E and M fields are generally 90 degrees out of phase in a plane I think)