The car chasis and bodywork are earthed, hence, so would any signal that it received. Nice try though
Rich's pretty much hit the nail on the head there too - your home hi-fi aerial is positioned once and stays there, your car is constantly moving about. The position of an aerial is pretty important because of the path the signal takes to it. Thats why bee-sting aerials are, in theory, so good. they can pick up vertical and horizontally polarised waves (caused by reflection/refraction of the EM wave), where as your standard "remote control car" aerial is designed to just pick up horizontally polarised waves.