You mentioned earlier that the mystery wire was "very thin". Is that because it was burned up so that little is left? If the wire really was thin to begin with it seems unlikely that a previous owner would have used it as a bypass around a damaged OEM main feed from the battery wire. I had thought that it might have been used for a battery maintainer, although that would have only worked if the disconnect switch wasn't used.
very thin gauge to begin with.. it looks like it fed across the seat plate into where the drive controls are.
there are some buttons and switches on the handle that have never worked, maybe it was an add on there, not sure?