The 12µH is (to the best of my logical knowledge of electronics and mechanics) there to suppress noise. Noise? Yes, from the alternator. The alternator produces AC that passes through a rectifier before going directly to the battery where all other devices acquire their power from. The radio will also have something similar in series with the battery, otherwise you will have a constant "whine" in the sound that goes up and down with engine speed.
I hope my answer is correct, or least on track.
Sputnik :idea: