I remember a thread where this was discussed: My Vantrue N2S Dual 2.5K Dash Cam with GPS installation
I went thru something similar, I have a mirror dashcam Akeeyo 360 (dashcam attached to the rear view mirror) that has a rear camera. I have the dashcam powered by tapping the fuse box described in the thread above. However I still had to run a wire to the rear hatch door for the rear camera. Basically I did not have to completely remove the trim pieces, just pull them out a little bit to allow running the wires. Especially the A pillar was very difficult to completely remove because it has those locking clips from hell (this is intentional so that the A pillar trim don't fly to your face when airbags deploy), and it was quite difficult, but doable. Also about the A-pillar part, remember to run the wires behind the air bag so that it does not block airbag deployment path.
Running my rear camera wire it was not too difficult through the side trims (Between the doors), just be mindful again about the airbags. Above the doors, simply pull the rubber seals and pull down the headliner a bit to run wires. Towards the side-rear, the side-rear trim pieces also easy to pull off a little bit all the way to the back headliner. Now between the rear headliner and the hatch door this could get tricky. For a nice clean install, I run the wires through inside one of the two wire boots, but that was not easily achieved. The wire boots are quite small and angled. Also my wire happen to have a right-angle mini-usb connector that makes it even more difficult to fish through the already tight space. I should have spliced it, but what I did was remove one of the boots first and run it that way but note that the boot clips/snaps are extremely weak so I managed to break all of them darn boot clips. I had to superglue the broken clip pins back, fortunately they are still holding up so far. Other than that, the rest of the trim (the plastic panel above the hatch door) unsnaps pretty easy.
The end result is pretty good, no unsightly wires and the rear 12V socket is still available for use for something else.