1.The pillar covers beside the hatch are easy to remove. I think there’s two(?) small bolts that hold them in along with a sort of w shaped plastic clip. Once the bolts have been undone, you just gently wiggle it away from the body and the w shaped clip will release.
2. The third brake light should have a screw on either side of it holding it to the body. And that’s in your tailgate door jamb area right above the weather stripping. I believe they are torq screws. Once those are removed, lift the light housing away from the body and gently pull the rubber grommet that covers the brake light wire out of the body. Unplug the harness and the brake light should be free from the truck at that point. Make sure you tape the other end of the brake light pigtail to the truck where the hole for it is, so you don’t lose it inside the hole.... I made that mistake when I was swapping my Walt third brake light in and it took me a minute to get the end of the pigtail back out so I could reconnect it with the pigtail on the Walt third light.... that was fun... lol