1983 GS1100ESD wiring harness modification?

[posplayr]

You persist in describing the configuration without providing the pertinent information of actual build that a diagram, schematic picture would provide. That being said it should be clear I can't make any more specific recommendation that has been posted for years.
my recommendation is to reassess your wiring at your wiring and ask yourself where you can put the SPG so that the wire from the SPG to the R/R(-) is shortest.
you seem to have configured for the shortest path between SPG and the battery(-) which is not ideal snd counter to what I have described multiple times.