single yellow wire from voltage regulator on my 1976 dt400. The lead doesn't seem to reach any connectors(mounted on air box by carb.) Says it connects to the headlight. But that's a long way from mount.

Splice or in some way connect it to the yellow wire coming from the stator.
You could also extend the wire to the headlight bucket. If you do this, it has to connect to the blue wire that feeds the dimmer switch.

On the 76, there should be a socket very near where the regulator is mounted. That was stock on that bike, so the wiring harness is slightly different than on a 75. If you can't find it, someone has removed it. It is color coded, so it should be yellow.

Sorry for picking up this old post again, I found it, searching for connecting possibilities of my Regulator. It´s mounted on the air filter box of my DT 250 B. For me it´s not clear, where the yellow cable from the regulator originally joined the main wiring harness. Was it installed into the joined connector behind the air box (above my thumb in the photo)? Or somewhere else?
My regulator cable leads from a yellow into a brown, what leads nowhere. Think the brown is not right, was installed from previous owner
I don't want to install it onto the headlight, but have read the regulator is more efficient, if it´s located with more distance from yellow stator cable