I haven't played with one of these, so treat what follows with caution.
From the picture here, the silver-coloured contact is the earth:
http://parklifeparts.co.uk/store/index. ... ge&pID=713
as it attaches to the axle locknut. That doesn't look like a very reliable way of making contact to me, so what I would do is either:
1: run 2-core cable front and rear to avoid relying on the frame as a contact
2: just run all the power to the front light (again, with a 2-core cable) with a 3W halogen bulb. Use a battery-powered LED rear light.