April 8 2004 - This covers the wiring harness of my bike. When I got the bike the wiring was a mess. It had been cut up and modified. Not much remained of the original harness. My bike is a military bike with a civilian headlamp. The difference between the civilian headlamp and military headlamp are two main items as far as the electrical system is concerned. The civilian headlamp has a generator charge lamp, and a fused circuit in which the fuse is externally accessible. The generator charge lamp is a handy indicator as the condition of the electrical system. The rear cover of the magneto was also different for the two military and civilian. I had the cover changed to the civilian version when I had the magneto rebuilt. The civilian cover has a terminal for connection to the charge lamp in the headlamp. At first I purchased what was supposed to be a reproduction wiring harness. After examining it and talking with other R12 owners, I determined that is was not going to route the way I was hoping and it had many extra wires, which was going to make it difficult to get through the headlamp and grommets in the frame. The particular harness I ordered supported a variety of pre-war/war BMW's, it was a generic harness. I decided that it would be in my best interest to construct my own harness(s). So I returned the generic harness and drew out a diagram of my electrical circuit as it applied to my bike. Next, I ran the individual wires allowing for extra length. After all of the individual wires were run, I put the various legs of the wiring into black heatshrink. Below is the diagram for my custom harness.

 Below is a photo of the main harness. I also had two more two other small wiring harnesses. One two-wire harness connects to the horn and passes through the grommet in the right side of the frame. The other two-wire harness is for the magneto. It passes through the left grommet of the frame near the steering head and runs under the gas tank (front clamp). The main harness is below. The left end is the headlamp connection (4 wires). Next, is the battery connection to the magneto (red wire). The next two small branches are for the battery (two wires) and the brake lamp switch (two wires). Finally you get the taillamp (3 wires, which include a ground wire). 

The main harness, which runs down the left side of the frame beside the tank, comes out near the rear fender. This is the junction where the three branches split. The battery wires come down and is visible in the foreground (red). The little bit of harness that is visible between the two is the brake switch harness. It runs down between the center frame channel and the rear fender and comes out at the bottom of the fender. The last branch is the taillamp harness and runs up the rear fender.

This photo shows the brake switch wiring that is running inside the lower right rear frame channel and up to the brake switch. I made a small plate to mount the brakelamp switch to. This plate mounts to the inside of the rear footboard using the footboard bolt.

This photo shows the magneto wires. The red wire (direct connection to the battery) is coming out of the main harness, which runs in the left frame channel. The remaining two black wires which are in front of the rear cover are the second harness which runs underneath the tank and comes out of the left front grommet.

Here is the taillamp harness running across the rear fender.

Again, here is a shot of the harnesses coming out of the frame for termination into the headlamp. To the right (in this photo) is the main harness and the two wire mag harness. To the left is the two wire horn harness. Notice lots of extra length. Its easy to trim off excess, not so easy to add if you are short in length.