My cap brake light is wired from behind the taillight on drivers side. The wire tap is snaked upto the cap with a inline 5A fuse and some bullet connectors for easy disconnect when removing the cap. I take my cap off about twice per year. The Dome light is also has a power tap from behind the taill running light, then the cap dome light and brake light share the same ground wire.
If you are wiring the brake light, just add another wire for a handy dome light.