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Wiring 3rd Brake Light on Truck Bed Cap for 2015 Chevy Silverado If you follow wiring harness feeding the 7-way trailer connector from the bumper up toward the spare tire and peel back the wire loom, you should be able to find an additional blue wire in the harness. The blue wire carries the 3rd brake lamp signal for your bed topper. For the interior dome light, you can get 12 volt power from the black wire feeding the trailer connector. You'd want to install a switch on the circuit like part # PK34513. One of the wires would be power, the other ground. You could also call the topper manufacturer, and they'd be able to tell you.