Well, I should have taken a picture last week when I found the exact wire that you were looking for.
Anyway, it is quite easy to find. Lift the tank, and then up near the front you will see a big wire sleeve coming from the lights through the rats nest of wires and then underneath the tank. There is a coupler with, if memory serves me, three wires. One for each the low beam and the high beam, and then the third I guess is a ground (somebody smarter might correct me).
Anyway, I found it in about 30 seconds by using a continuity tester. One of those little poker things with a light bulb that lights up when you find DC power. The wire from the lights was obvious, then I verified the high/low wires by flipping the highbeams on and off.
I ended up not using it because I needed a circuit that was on all the time to run the sidecar headlights, and I didn't want them going on and off when I flipped from low to high beams or visa versa.
Anyway, get yourself a continuity tester, lift the tank, and poke the end of the tester into the coupler up there and you will find it quick.
Not sure if that helps ... but that's how I did it, and I'm not exactly an electrical genius (as evidenced by my recent F'up of melting the entire tail-light circuit).
PS: I also followed the advice of a fellow forum member and used a relay. I melted the first circuit, and didn't want to melt a second. The relay worked great and was quite easy with the wiring diagram that somebody sent me. I have a copy of the diagram still if you want it. Just holler.