The cable between the front light and the back light is two-core oval mains flex. Traditionally bell wire is used, but mains flex is tougher. The cable is looped around the head stock with just sufficient slack to allow the handlebars to be turned, and then along the underside of the down tube and left-hand chain stay. From there it goes up a rack stay to the rear light. Much use is made of zip ties to keep it there. Do it right and it wont really be noticeable. Note that the rear light also uses spade connectors. Mains flex sourced from B&B Electrical Wholesalers on Ribblesdale Road. Maplin also sell it when they actually have it in stock (don't believe what their computer says), but they charge more. Three metres will be sufficient for a standard bike.
