There have been several posts in the forum about this very subject. Yet there is still no sure fire way to have this happen that I have seen. If you look around here you can find many great Ideas that may work for you. There are posts here from everything from fabricating your own from a tin can with some led's to purchasing one from one of the members here...Unfortunately I don't remember which thread has this info.
I'm still kinda a noob here but I run mine from a light I made and it runs off a 6v battery. Take a look at these they might help give you some ideas.
BIKE LIGHT "Commuter" under 5 bucks By Veggiecycle
BIKE LIGHT 500 Lumen "Mt.Bike" for under 10 bucks by Veggiecycle
ESR 750 electric scooter - Go-ped - Electric Bike or Scooter - Batteries Plus
Sorry for the lengthy response, I tend to ramble. But as the members here helped me I want to try to help others. Sometimes that takes pointing out a new possible solution. If you are determined to use the white wire then you may find the solution for us all but until then, have a look at the motorized bicycle Forum of knowledge.