A maze with no exit (brake light wiring)

[pekingduck]

Thanks, pekingduck, that makes sense. Except that with the Honda OEM light, surely it completes both circuits? How is that supposed to happen with the new arrangement?

Yes, the green ground wire is necessary to complete both circuits on the stock taillight because it has a plastic taillight bracket rubber-mounted to the stock steel fender. (The chrome plating on the plastic is actually conductive).

But the aftermarket light gets its ground, assuming the base is metal, when it is bolted solidly to the steel rear fender, in turn grounding to the frame.