There is at least one j-box under the bed that you could tap into. I know some of the guys have tapped into that for their bed lights. When my dad did the shell on his 72 Datsun PU, he used a trailer 4-wire harness, with the connector near the shell where the wires were hanging, but this was for the clearance lights, interior cargo light, and exterior hatch light. Even when he bought my 91 Nissan King Cab, he did the same thing, or had the place that he bought the shell from connect it. Neither one had third brake light, I don't think.