This is how I did my Dashcam,
1. I first powered it up from the power socket in the front in order to place it where I wanted it. Mine has a screen so I could see what it saw, and made sure I put it as close to the rear view mirror housing as possible without "it" seeing the black corners of the housing itself.
2. After much jiggling and minute (bordering on anal) adjustments, I was happy with a position that hid it from my view whilst driving and made it close to the rearview mirror that it would be stealthy from the outside.
3. I then fed the power cable into the rear view Mirror housing and went around the inside of it, then up and into the headliner above the driver's side
You can just poke it in there with your fingers, the headlining is not sealed shut up the front so there's more than enough leeway to poke a cable under it.
4. On the corner the cable comes down and behind the side pillar air bag trim, I personally didn't have to remove the Airbag sine and unscrew the cover to put it through, for me I just managed to pry it only so much to poke the cable through and then it just slowly returned back into place.
5. Next I just ran the cable inside the door frame seal (there's enough room for a cable to go in there, around the centre pillar (making sure I poked it inside the cover, again, wasn't too much of an issue) and then round the seal for the rear door.
6. This is probably the only part I am not too fond of, I couldn't put it underneath any trim near the backseat, and I didnt want to take any trim off either, so I just made it follow the join between the black plastic bottom half and the soft cloth upper half near the seat belts
7. Lowered the seat back, passed the cable through then closed it up again, then ran the cable behind the side wall trim, you can poke the cable in the gap between the wall and the plastic shelf until the cable ended up in the fuse box area.
Any extra cable was just left in there (there's wasnt much left from a 3 meter cable) and then just extended what was needed to reach the trunk power socket as shown here
And voila... Dashcam done.