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Go to the back of your truck and crawl underneath. Follow the wiring for your trailer connector back up behind the bumper. You should be able to locate a green wire that is labeled as CHMSL or Center High Mount Stop Light. That is the wire that you will tap into for your brake light on the canopy.
If you have an interior dome light you can tap into the black wire going to your trailer connector. That will be 12V+ whenever the key in the truck is on. That wire will not work if the key is in accessory or not in the truck. If you want to be able to turn on the dome light at any time, you will have to run your own wire from the battery back to the canopy.