Okay got some pics, mines 3rd gen but the exact same thing can be done with a 5th
Chase/camp light
Sheathed chord from light can be squeezed though this gap. Open and close the tailgate slowly while trying to feed the chord in from edge of the tailgate, you'll find the sweet spot where the gap is big enough to pull it in.
My light came with a deutsch connector pigtail, I strapped it to the tailgate hinge just to prevent it from moving around in there. Plenty of space even when closed. The male plug is from the supplied harness which made things super simple.
Heres where I unsheathed the harness where it goes under the weather stripping.
I have power running from the battery up front to this little box behind the passenger wheel well, the ground is a nearby bolt in the body with the paint sanded away. Inside is just a few positive and negative terminals, that's where my harness is getting its power and ground. You don't need the box, a single wire would do fine, just get power to the trunk somehow, and find a ground that fits your harness.
You can also see the other side of the harness going under the carpet to the front of the car.
Harness wasn't long enough to bring to the dash, so it snakes under the carpet and trim to the B pillar, then under passenger seat to the centre console instead
And this one's pretty self explanatory.. switch lol
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