Black is always ground on the Ducati wires. White should be the low beam and the yellow with black stripe should be the high beam. If you look closely at the wiring diagram, it usually shows a smaller filament for the low beam and a larger one for the high beam to distinguish the two. Unfortunately they don't do the same thing for the tail light.
When I put grip heaters on my Monster, I tapped into the parking lamp circuit. On my '03 it's a solid yellow wire. And before you ask, the parking lamp is on whenever the headlights are on! Don't ask me, I just work here. My guess is that if your headlight fails, you have the parking lamp for some illumination. It works so I'm not really questioning it.
I originally tapped into the turn signals but then realized that they were just that, turn signals only. I have the heaters wired up to the headlight. THe headlight works but the heaters are not. Time to pull out the volt meter! Thanks for everyones help.