I recently bought a 2019 Ranger that came with an ARE topper. The topper came with an upper, or 3rd brake light. What it does NOT have is a dome light. I have attached an LED dome light which has an on/off switch. I most often use the dome light when nothing else is running on the vehicle and could run about 10' of wire from the positive battery terminal along with a 10 AMP in-line fuse. As long as I turn the dome light off when not in use - no worries.

It sounded simpler to tap into the wiring of the rear brake light instead of running cable from the battery. However, once I looked behind the taillight there are so many different colored wires going to the rear brakes I have no idea which one to tap into. The 3rd brake light uses the red colored wire shown below with yellow arrows..

Yellow arrows = 3rd brake light wired into driver's side tail light

Also, if I tap into the correct tail light wire will the shell's dome light be available when everything in the truck is turned off?
Thank you

 

The issue you're going to run into is there must be power sent to whatever circuit you're tapping in to. At the rear of the truck, that's the parking lights, brake/turn and license plate lamp. None of those will be powered up when the truck is just sitting.

I put LED in my topper, but intentionally kept them off the vehicle battery. 12v LED light strips powered by 3 18650 batteries in series.

 

Just run it to the battery and be done with it. If you leave it on, then the light being on should be a reminder you need to shut it off. If not, then I'd invest in a little jump pack.