There's a dedicated CHMSL (center high-mounted stop lamp, aka 3rd brake
light) wire that's ran to the back of the truck specifically for aftermarket
items. It's a white/tan wire you're looking for. Look for it in the wire
loom that feeds the rear lights. Simply tap into that, and run to your
aftermarket brake light. Usually, the aftermarket light will have it's own
connector (so that it can be disconnected, and the cap removed), and provide
it's own ground.
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