I couldn't find an exact schematic for your bike, but found one for a 83 zx750A1.
That reg/rec has the same wire colors.
White goes to always on +12V.
Brown/white goes to switched +12V
This wire may operate as a voltage sense wire. On the stock system on a 83zx750 it ties into the brake light switch, the horn, and the turn signal relay.
You are eliminating these paths with the m-unit. So you may need to wire it to the light circuit or one of the aux outputs on the m-unit. Someone else may be able to clarify this.