3rd brake light wiring

Does anyone know if there is a company that makes a block-off plate or wiring harness to tap in to the 3rd brake light/cargo light? I'm going to be building a headache rack which will mostly block the 3rd brake light so it will have integrated brake lights and white reverse lights similar to the magnum rack below however I don't want/need the reverse lights every time I back up so I would like to take advantage of the existing wiring and switch for the cargo light that is built in to the factory 3rd brake light. Which brings me to another question; what kind of amperage can that cargo light circuit handle?