I'd like to offer some advice after doing this install yesterday, and also seeing guides and general chat about how to do this that could be DANGEROUS.....
Firstly, the safety part..
The A pillar of most modern cars contains an airbag curtain which drops vertically down at very high speed, protecting you or your passenger from striking their head on the glass or intruding object.
Most people seem to run the cable along the headliner at the windshield to the corner (fine). The problem IMHO is that majority of the advice I read is to then jam the cable between the plastic trim and roof liner, and cross underneath the airbag (!), then run down the rubber trim the length of the pillar. By crossing underneath the airbag, you are effectively blocking the deployment of the airbag properly in the middle of its width, OR during deployment the bag will rip out your cabling and possibly dashcam, sending this towards the seat occupant at great speed....
So, don't do this. Please.
The plastic trim on the A pillar seems to be near impossible to remove, but if you can, the above method should be okay so long as you can cable above and behind the airbag carefully.
What I did however, was run the cable down the windscreen, to the seam in the dashboard, then carefully pushed the cable using the blunt back of a teaspoon into this crack, completely concealing it, and then allowing access via the side panel to the fuse box.
GROUND:
When you open the dash side panel, there is a convenient bolt (with nut) that I used for ground - simply find another but with the same thread and add it on, trapping the ring securely.
REAR CAMERA:
For the rear camera, I opened the boot and gentle loosened the headliner clip in the centre. I then used a length of what we call 'yellow tongue' (flat firm 3m piece of plastic for pulling cables) and pushed from this end all the way along the to roofline until it popped out at the windscreen, then taped on the rear camera and pulled the plastic back out. This was actually very easy!
I fed the cable through the rubber elbows to prevent water leaks - but I did break both clips doing this, and I'm still not sure how to get these out without doing so, so please use caution.
FUSES:
I used fuse taps for both the ACC and 12v wires - the 12v I got from the cigarette/12v fuse, and the ACC from the USB fuse (mini fuse).
Hope this helps!