Join Date: Aug 2017
Location: Perth, W. Australia
Can't help with specific M750, a workshop manual electrical diagram would show source of yellow wire.
I do have M900 year 2000. I'm quite sure the 750 is similar. On this, the source of yellow wire is at the ignition switch itself.
There are 4 wires on the 900 coming from key/switch.
1. Red : from ignition relay and fuse box.
2. Orange : from ignition relay.
3. Orange/Black : going to left handlebar switch unit.
4. Yellow : to headlight, taillight and plate light.
This is for when lights on at all times models, some Euro models have additional switch on handlebar for turning lights on/ off. If so, and no power in yellow, then testing of the Orange/Black and or handlebar switch itself may be needed.
Use a voltmeter at key switch to test yellow wire with key on.
Hope this helps,