I'm installing rear turn signals on my 1974 DT250 and I was wondering where the correct location is to install the grounding wire   A parts manual shows there is a wire with a ring terminal on the turn signal stem.  I would like to connect the other end of the wire to the factory stock location. 

Thanks for any assistance.

Without being able to easily find a diagram online, it stands to reason "yamaha reason" that there must be a black back there.
Might be a black with a double connector but possibly a separate black for each side following the flasher dark green & dark brown wires so could end up the same length. There must be a black there to earth the tail light but i can't imagine it having a triple connector for that & flasher earths which is why i think there could be more than one black.
Theoretically you could earth those flasher wires anywhere to the frame as all lead back to the loom earth at the coil mount on the frame on older models i'm thinking of.
Someone with updated knowledge will put you right shortly.

Yes, there should be a black double female coming out of the main harness. Often this is chopped off and has to be reinstated - I had to do this on my own bike.

If it is not practical to reinstate it, then you could attach a ring connector to the other end of the earth wires and bolt these to any convenient part of the frame (make sure they connect to bare metal, not thick paint).