I know there's been some previous threads discussing these type of cameras before, but I thought I would add one that's specific to a Blackvue dashcam install in the W205. Below is an illustrated story line of how I did mine .. hope it helps. 

Equipment as follows: (I won't repeat specs since Google has that side of things well and truly covered)

  • Blackvue DR600GW
 
  • Power Magic Pro (also from Blackvue; This unit will allow the camera to keep recording once the car is turned off and it also prevents the battery from being drained)

What you need:

  • Fuse taps (Blade type; x2)
  • Wire rimpers
  • Thin plastic spatula (even credit card will do)

Steps:

 

  • Mount the camera
  • Run the cable through the top of the headliner and then out towards the side. Using a small plastic card to gently open the edge is all that's needed. Once you're on the side near the door you can follow the rubber lining (red dotted line is where I've run the wire)
  • Keep following the roof lining all the way to the back. The joiner near the B-Pillar is easy to open up enough for the cable to pass through; there's no need to undo any clips or lining completely. 
 
  • Once at the back, follow the upper line of the C-Pillar. I found that the lower segment where the air bag meets the seat was too tightly clipped joined so I just ran it along the upper part all the way to the back screen and then back around towards the seat where the rear speaker is on the shelf. 
 
  • From there it's an easy path along the lower edge of the seats into the trunk. The blackvue ran out of cable about 3/4 along the rear seats but that's not an issue. The PowerMagic has plenty of cable length to make up for the shortfall. The 12V socket to plug in the unit can hide under the lower edge of the seat (I used some adhesive tape to secure it from rattling around within the seat cavity. 
 
  • Now remove the rear liner covering the fuse box. You need to make three connections with the PowerMagic Pro: 
    - Yellow wire goes to a permanent batter 12V connection. I found this on one of the lower right fuse points that were unused. 
    - Red wire goes to an Accessory switched 12V connection. I found this in position 445 (labelled Socket 12V in trunk)
    - Black wire goes to ground; I used an M8 chassis bolt located near the upper right corner of the fuse assembly.
 
 

Tidy up your wires, set the PowerMagic timing dip switches as desired and plug it all in. Done! ..