Hmm, I think you are just going to have to get a meter and follow the circuit until it becomes obvious. I'd start at the bulb holder and work back from there. My guess is it will be somewhere at the front of the bike, easpecially thinking about the number of folk who have reported problems from dodgy ignition switches - might be worth fumbling around with the wires under there to see if it does anything.
I don't know if there are any wiring diagrams availble online anywhere.
HTH