I show how I route the main wiring harness after I installed the headlight shell. Previously I replaced the main harness with one from Euro MotoElectrics.
Here is the document showing the details and a short video.
VIDEO: 1983 BMW R100RS Routing Main Wiring Harness
Below are a couple of the pictures from the write-up.

Front Brake Light Switch Sub-Harness Plug (GREEN–Red, GREEN-Black) & Clutch Switch Sub-Harness Plug (BROWN–Yellow and BROWN )

Main Wiring Harness Coil Power Terminal (GREEN–Blue) Goes On Same Terminal As EME Optical-Electronic Ignition Control Unit Power Wire (RED)