where does this wire go? KZ750 Twin

I believe I've seen yellow / black-stripe used as ground for something on a Kz. Maybe on something more elaborate like a police bike. I'm just not sure it's always used for ground only.

Yes, black / yellow-stripe is the general rule for ground, but I've seen black / white-stripe as well for small parts of the harness...possibly it was a Zx (thus not a Kz) ignition related probably.

EDIT: Here is an example on a 550. Notice the accessory leads use a yellow / black-stripe wire for ground.
s3.amazonaws.com/gpzweb/Mnls/Kz400500550...kawKz550C2_UsCan.jpg

The original poster will want a 1983 Kz750K1 wiring diagram, as it seems that is most likely what bike he has. The diagram will probably show what his wires go to.

I went through that wiring diagram and what I have is not on there. The good news is I figured out why my right front turn signal light is not working (ground was not connected in the headlight bucket).

If you mean the KZ750-B wiring diagram, then it's incorrect for the 750-K1, as the 750B doesn't have an acc fuse box. Other stuff is probably different as well, so try to find the correct wiring diagram or get the Kawasaki service manual that covers your model.

As for the wire color >> you said: The black/yellow wire coming from the inline fuse box shows 12v without the ignition on
Is the black/yellow wire in the photo above? In your pic both wires from the acc fuse box appear to be white, not yellow, and I assume they're white-blue & white-black. They both look white-blue on my computer, but one probably should be white-black.

Both acc terminals probably should be hot as they are usually wired directly from the battery pos (+). You already confirmed the one in the pic & as already mentioned, the other is probably inside the headlight bucket.
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It's hard to say what happened with his harness, but a wiring diagram would help to figure it out. Without one it would be hard to guess.

It's possible his accessory leads are like those of the 550, where there is a yellow wire with black stripe as ground. But then there shouldn't be voltage on it.