you will actually hear junk from the alternator in the AM band if the voltage regulator(s)/rectifier goes bad or a ground gets corroded... the lead acid battery provides a nice smoothing function (almost ideal voltage source) since the alternator and engine have a low impedance bond to the negative terminal (system ground). the low frequency spectral emissions (very low harmonic content) from 1 alternator and associated circuitry is a much simpler problem than all the computers, ac inverters (induced high frequency currents running around everywhere), wiring harnesses, etc. that all act as rf generators/antennas.