I have a mystery. I've looked at wiring diagrams until my eyes have crossed and I can only assume this stray wire in my headlight bucket is for a headlight on/off switch that didn't exist in Canada because we have daytime running lights laws. It's a 1980 DT100, and everything works. Lights, gauge lights, everything. I could just leave it alone but this stupid blue wire is staring me in the face every time I open the bucket.

Picture of the errant wire:
imgur.com/a/0jMUlfX

Do you have a hi/lo switch?

I think its for an oil tank low level float switch if you have one fitted. Maybe a comon wire loom for various model of which not all run a float switch. My guess--ok i cannot lie--i googled haha.

I think I figured it out. Ran over the harness with a multimeter to test continuity and re-read the DT100D/E/F wiring diagrams. I think this is another Canada model oddity.

The gauge lighting was wired to the blue wire running to the headlight. According to every wiring diagram I can find that wire should be wired to the mysterious Blue/White wire, which also runs to the tail light. None of them show a double socketed connector for the headlight to allow this.

So either the previous owner expertly wired up a double bullet socket connector (it looks totally stock) or Yamaha wired these up a bit differently per country. Maybe because of daytime running light laws. In it's current state the gauge lights up when the engine & headlight are running, plugged into the mystery wire it lights up when the key is turned on, regardless of the engine. I like it lit all the time, like the tail light, so I'm using the mystery wire.