Hey everyone , I had to go with aftermarket headlight and 4emove factory plug. Black should be ground but can't find a black one that checks to ground in the bucket. Also can someone please tell me what the ohm reading for lighting çoil should be ? As always thanks for your help. 1980 dt125 usa model.  Problem solved , bad ground connections  thanks for the help. 

If we knew the year and market destination of your 125 we would be able to help you.

1980 dt125 usa
 

Lighting coil. 0.19 ohms at 20C on the yellow wire.

Run a black wire from a ground on the frame.

Do I mean yellow wire to red probe and black probe to ground and meter set to what ?

Headlight runs on AC and engine must be running.

Yeh , it's not working  only light burning is little blue one in the tach. 

Stock headlight had 3 wires, yellow, green, and black.  Black was ground.  If it's not anymore then maybe that was why original headlight didn't work?

Little blue light (high beam warning) needs a ground wire too and it connects in bucket.  If it works (only with engine running), stator must be putting out something and there must be a ground?

This model is a standard 5 3/4" headlight with many options for replacement without cutting up the wiring so hard to help much more as I don't know what you or someone else has done to the wiring and what you're trying to do now. 

Black is ground on headlight. I ran a ground to the frame for it. When I get it started back up I'll figure it out. Other headlight wires are correct per wiring diagram. If I was certain of the coil resistance measurement it would help save time. Thanks
 

Factory Service Manual (and 1980 supplement) for that model is in the Tech Library.  Specs and test procedures are there.