I don't remember where I picked up power, but I did research the wiring diagram and picked one of the fuses that is less critical. If the grips blow the fuse, I still want to be able to ride the bike. So pick a circuit that has less critical functionality - like lights.
On my kid's bike I hooked it to the headlight power and tell them not to run the headlight and the heaters at the same time. That bike doesn't have the power to drive both simultaneously like the Berg does. We don't ride at night, so it has worked out okay.