1976 DT400.

There is a yellow/white wire in the main harness that goes to the rear of the bike. It is not in the wiring diagram and is not related to the voltage regulator. It goes into a plug near the gas tank, turns into a white wire, and goes up to the left handlebar light switch array. Any ideas?
 

It might not be relevant, but what country are you in?

USA. Could it be the blinker noise requirement?

Make a couple more random posts (maybe one to say "hello" and another to say ok to this post of mine) and then you should be able to post photos of the item, although you'll need to use a third party host, such as Imgur.

Yamaha diagrams are not always perfect.  And are you sure you have the DT400C diagram?  Changes were made between the B and C model.

Yamaha was fairly consistent on their color use though.  (CDI harness wiring on CDI bikes might use same colors, but those wires only run between the CDI components)

Yellow/white is generally used in the main harness for the AC Headlight power circuit.  (There is a yellow/white wire shown on the DT400C diagram)

Solid white is generally part of the AC power circuit.  White powers the rectifier which outputs DC to charge the battery.

P.S.  Not sure what a "blinker noise requirement" is but as far as I know there was no such requirement in the US for 1976 models.  Turn signals generally use "browns and greens" for their colors.

 

Was in the diagram I looked at,  was a voltage regulated a/c power supply to handlebar sw that supplied on off power to hi lo beams.  

the white is there but is sw from g/r mag power with the white latter getting rectified.