^ Wrong.92gt5spd said:headlights dont draw power from the alternator. They draw from the battery. 13.5 - 14 volts is what you should read. If not you might have a failing battery on you. You might have something hooked up wrong or something. Think about it. If you once had a radio in that spot and i'm sure you had your headlights on w/ your radio on at one point. And since a radio runs on 12 volts...then the radio wouldnt have turned on or would have cut out when u flipped on ur headlights.
When the alternator is working, ALL power is pulled from the alternator first, untill the voltage is pulled below 12 ( at that point the alternator is at full load) than the remaining power is pulled from the battery.
If your head lights some how pulled power from the battery and not the alternator you would have a dead battery in a matter of time.
Most stock and aftermarket radios have a operating voltage range from 9-16 volts. So it's not gonna cut out from the draw from the head lights.