Hola R-man I'm not familiar with this layout but suspect is not rocket science
...
I daresay it's nothing more scientific than a crappy earth, as the handle bar isa bit isolated from the rest by the headstock bearings/grease.
Start at the Maggy earth cutout wire (what end-cap do you have) and progressively earth it out up the wire...if yea kernow whut ah means ??
Maybe add a straggly wire to the end cap that can be easily earthed (when desired/needed), and go from there