SON is nice enough to sell these little units for aftermarket modification of headlights. Soldering in this guy on your own can be a little tricky, only because the space is cramped in this tiny box. See:
A link to the instructions as a PDF for downloading/printing, the unit will come with a print version of this too:Coaxial-Junction-Box-Soldering-EN-2018_A5
You can add this Y-junction inline to any headlight that doesn't already have some way to funnel off power from the hub. But it works best using the coaxial wire that the SON lights have. Its a bit beefier and bigger in diameter and the y-junction box is designed around it. And obviously the instructions are shown using their wiring so it'll look different if you use a different brand.
Don't use this to connect your taillight to the hub's power, that's not what it is designed for. It's intended to allow you to add an adaptor to charge devices like a GPS, phone or external battery. If you would like us to do the work for you just drop us a line and you can ship your headlight to us and we'll do the install, then send it back! The SON Edelux II headlight is available with this unit soldered inline from the get-go. Pair that set up with a Sinewave Revolution that has a Male Coaxial plug soldered on and you're all set. No one said any of this was going to be easy!!