So the body of the DS light is the ground, meant to be mounted on a metal surface to pick up the bike's ground through the frame. LED or incandescent? If the latter you can swap for a two circuit lamp holder, otherwise you're holding a dedicated single circuit lamp. Most likely you could make it work on a two circuit by putting a couple of blocking diodes in line to keep the two-circuit lines isolated...that's more up Sandman or Phils' alley than mine...
