In this post we deal with the wiring of a rear SON light. Most of the instructions are already described in this product sheet from SON. What we however struggled to grasp, amateurs as we are, was how to connect the rear light to the front light. And this is important, as you most likely would like the rear light connected directly to the front light, so you can turn it on, off, or on auto-sense, with the switch on the front light. This thread explains how the end result should be. We try to summarize the steps needed below.
1. Start by ensuring that your headlight is functioning correctly.
2. Mount the rear light and ensure the included, and already attached cable, is long enough to reach the head light. If necessary, extend it, preferably by using a set of crimping connectors so the joint is sturdy and waterproof.
3. Out of the box the cable is attached to the rear light and has already been prepared for connection to the headlight, i.e. the wire endings have been stripped from the surrounding insulation. Attach a ring terminal to the the wire that has no insulation at all. Then attach a spade terminal to the wire that has light-colored insulation. Remember to slide three pieces of shrinking tube onto the main cable and two wires before connecting the terminals.
3. Use a hair dryer or a lighter and gently heat up the shrinking tube until its a snug fit around each of the connectors. Unfortunately we forgot the shrink tube around the main cable. But you can remedy that afterwards with some electrical tape.
4. Finally, attach the ring connector to the side of the bolt used to mounting the headlight. Insert the spade connector into the spade terminal just rear of the cable running to the dynamo. The spade terminal is quite small, and we failed to actually recognize it the first time we looked for it. Now you are all set, and the headlight and rear light should now both have power from the dynamo, and is controlled through the switch on the headlight.
If you need spares, SJS Cycles is a good place to look. They have all sorts of parts needed for (re)wiring, dynamos and lights. For way more in-depth knowledge on wiring, have a look at Peter White Cycles and their article on dynamo light wiring instructions.