What I did in the past: Using a circuit probe, disconnect the bulb from the harness, turn on the headlights select the high beams, touch the pins...use that wire. Double check your work with a helper switching between High and Low.
Not that you don't already know but for others:
I'd suggest running a trigger wire to a relay that way you can feed power thru a standalone harness. Don't forget to make sure the ground wire off the bar is done right. I've had to run extra grounding straps in the past to get good connections done right.