Use an insulated terminal box on the inner fender and supply power to it from the battery via a 4-6 gauge cable and a battery extender post.
Mount a relay for each item to be powered on the inner fender and run an 8-10 gauge wire to the relay and from the relay to the item being powered. Trigger the relay with the existing power wire that normally supplies the item.
Beef up the existing ground wire that goes to the inner fender.
You might check this link and see if it offers diagrams as well.