first of all, I just installed 2 brand new Naso Xenon Light bulbs (close enough to HID system)...so it was in daytime....i turned it on to have a look at it....it looks great....but somehow I forgot to turn off the headlights....and came back in the next day, tried to start my mustang...it won't start at all.....so i went to autozone, return the old alternator....got the new one....i installed it the new one....won't start....could it be the battery???? Let me know...thanks