Assuming that it's not a darker blue that has faded, then that is the neutral light.

Yamaha used the same wire colours throughout the model range, so looking at a wiring diagram for any Yamaha of that era will identify the function of each wire colour - eg all neutral lights are light blue, all horns are a pink and a brown etc. 

thank you

grtuncleal, I think he has a DT 360, not a 125. The DT 125 has a "Sky Blue" light for the neutral light. The DT 360 does not have a neutral light. Then again, it appears his wiring has been completely monkeyed with prior to getting bike. No disrespect to real monkeys in this case. 

Both the speedo and tach have blue wires for lighting. Connect them to the double barrelled connector in the bucket.

The source of the double barrelled connector is the lighting switch.
The blue wires light your instrument panel, but they also provide another function on your bike. You have a regulator. The regulator's yellow wire is connected to either of the blue wires. It will not work unless grounded.

Good point.

I guess it could be a light blue coming from either an incorrect tacho (my RT2 came to me with the wrong tacho, which had a neutral light), or an incorrect main harness.

day 2 failure had both signals working now only 1 I need a detailed schematic the one on the library isn't much

which one are you using?

I like the DT400B COLOR diagram. It works for your bike. 

 

I will print it thanks

Perhaps someone installed gauges from 125/175 or other model with a neutral light. 

no neutral light for sure...I have 1 wire that puts out 6 volts when I turn the key on what would require that?

Brown wire. The horn and front brake light. Turn signals are live too if switched on.