The piece you're adding to your bike is a Bosch D2B style magneto generator, right? Previously, your bike had a magneto only, with no electric lights or horn?

The D2Bs are an amazing bit of engineering. Bosch designed them in a modular fashion so they could be used on any one or two cylinder bike, regardless of timing considerations or rotational direction, by making just a couple of changes at assembly time. The magneto is not a wasted spark type, like those familiar with the post war bikes are used to; the armature windings are just not sufficient to power two plugs. The generator produces a nominal 45 watts (with a peak at 60), which was good enough for a 35 watt headlamp and an 8 watt tail lamp.

The magneto circuitry on your bike should continue to be correct. (That is to say, your kill button on the handlebars is wired the same way.)

The generator portion of the unit has three connections to the motorcycle: There are two screw clamps on the side of the domed, sheet metal voltage regulator cover. Although they are stamped with the numbers 30 and 51, they are internally connected. The third connection is the base of the D2B body, which must make good contact with the motor housing for a solid ground connection.

Basically, the positive wire from the battery (#30) goes to the #30 terminal on the generator. The negative side of the battery is bolted to the transmission case. The engine/transmission unit, and via the motor mounts, the frame, all become the grounding points for electric accessories.

The #51 wire leaves the generator and goes to the Bosch drum style headlamp. A separate wire should come up from the motor case to the headlamp for grounding, otherwise you are depending on a ground connection through the steering head bearings.

The headlamp is the distribution point for electricity to the rest of the bike. There are two wires that lead away from the headlamp. One is for the tail light and the other is for the horn.

The wire to the taillight supplies +6V, and is switched by the knob on the back of the headlamp. The tail light is grounded through the JN3 taillight's body to the license bracket, fender and frame. (If you wire a separate ground wire instead, with sufficiently long wires hidden away, then the JN3 can be used as a trouble light.)

The wire that goes to the horn is unswitched +6V, which means that the horn is always live. The horn has two screw terminals and it doesn't matter which one you use for the power side. The wire on the other side leads off to your horn button, which, when pressed, grounds the circuit to the handlebars (as does the magneto kill button).

Internally, the headlamp has a 3 position rotary knob: off - tail light - head light, in a clockwise fashion. There is a mechanical switch that selects high beam or low beam for the head lamp, which is operated by a short bowden cable to the lever on handlebars. I have seen two different forms of this switch and lever: in one, the switch is a pull on/pull off type and the lever just pulls the cable. In the other type, the lever has an over-center design and will hold a position, so the switch is essentially a momentary contact design (with contact held for very long moments ).

If you want to wire in a brake light, you have to modify the JN3 to accept a dual filament bulb. Put a second wire into the always-hot terminal at the horn, bring it down to the brake light switch you will have to install on the foot brake somehow, and then bring a wire from the other side of the switch to the brake light filament in the JN3.

Finally, for more (much more!) detail, you should visit Dietmar Nix's D2B web site: http://histor.ws/dmag/