cargo light for 03 cab and chassis

New install of a cargo light for 03 3500 silverado. There was a wire connection on the stock blank dash cover where the new factory cargo light switch was installed that contained all the connections for a fog and cargo lights. (I had neither switch for fog or cargo switches from the factory). I have a cargo light installed in the new headache rack and want to connect it to the factory wiring. Where Is the place to hard wire the lights too? Is it located in the junction box by the left foot or the fuse box under the hood? Do I have to purchase a relay / fuse...? or is it part of the center mounted brake light? Building the whole dump bed is easier than chasing electrons...