Well, you have more than one way to get power to the back window.
1. The interface cable is actually a splitter. If you were looking for a splitter, it would be easy to recognize immediately. It was designed that way. Since 2013, the car had that 4 pin connector in the speaker grille. It was designed to provide power in EU to an intrusion detection unit. Finally it was offered in the US. $350. But before the intrusion device was available, we used it for dash cams. Once the Intrusion device was available, everyone had to unplug the dash cam to plug the intrusion device in. Then Teslatap and I made the splitters. Allowed the Intrusion device to be plugged in and the "T" connector tapped power too to the dash cam again. (Some of us, used that black connector on our pigtail to make it look factory. So some splitters had one blue and two black connectors.
Answer your question, If you don't have the intrusion device, you could use both sides of the splitter (not the blue) to feed to dash cams and run the power wire from the speaker grille down the side of the car under the headliner to the back window.
I have OCD. 2. I tapped into the liftgate's controller power and used a heavy gauge wire, about 14 and added a 4 micro fuse box in the corner of my trunk. Now, I can add other 12V devices in the rear from the fuse box. From the fuse box, I ran wire up the C pillar, through the body, through the rubber boot at the liftgate, and through the liftgate body to the center of the liftgate window. Fused power that way.
I think down the road, not juse what's in front of me.