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I took a look at the wiring diagram in the service manual... doesn't look like there is anything which is a pain.
As Airhead; you can wire into something with high capacity but I would go with @skinny; the low voltage systems in the latest oxford grips are actually pretty good so you're unlikely to kill the battery (assuming it's in good condition).
They're as simple as undoing a battery terminal.
What bike do you have? Most have an aux socket or line somewhere which you can tap into...