There is a lot to read regarding the infamous 'white wire'.
CTripps is correct. Either cap it off or remove it from the magneto coil. It will give you 6 to 9 volts and only about 2.5 watts. That isn't enough to power a light bright enough for night riding.
Here's some reading for you on the subject > http://www.google.com/cse?cx=partner...ire&gsc.page=1