Here are some products. I use the former with wires in parallel with light.
If light is on, USB does not get as much juice. I do not know your extra leads from your light but I would guess they are for a rear light. In other words any device attached would share power.


http://www.thinkbiologic.com/product...icro-usb-cable
http://www.peterwhitecycles.com/ewerk.asp
http://www.amazon.com/gp/product/B00...A19K2B0ZZ0HPUD