Some bottle generator LED lights will work with a single conductor wire if they are grounded to the dynamo or to the fork and if the bottle dynamo is also grounded.

I mentioned above that I bought some clearance priced bottle dynamo LED lights years ago. I put one on my errand bike when I bought a cheap dynohub wheel from a bike charity. Both the dynohub and the light are grounded to the fork, thus only one single conductor wire needed to power the light, no taillight on this bike so the wiring is amazingly simple, a piece of single conductor wire less than a foot long is all the wiring I used. No switch on light, it is on at all times when rolling. In my case, the threaded rod that I used as the light bracket is how the light is grounded to the fork.



If I ran a taillight, the tabs are on the light for that purpose, but did not have a spare taillight at that time to use.