let me clarify. Like for the head light. I leave harness intact. Get a solid state relay that will do 12dc for the load and be triggered by dc (never seen one that is triggered by ac). Hook a rectifier up to the headlights regular + and - terminal. Then hook that to the the solid state relay trigger. Meanwhile, run a wire from a battery and then you can still keep the harness pretty much intact and not have to mess with new grounds and so on.
I can charge my battery @ work and its only 5 miles away.
get a larger 20amp version for the headlight, a smaller 2 amp should world for the rest.