Much tidier but more longwinded method is from the centre armrest 12v socket fuse which is located in the rear SAM [fuse box] in the boot/trunk.
Ha,ha], from camera, across top of windscreen and down behind 'A' pillar, behind end of dash, down behind rubber door trim then under inner sill cover, behind carpet and up to area behind rear side speaker and into boot space where it can be further concealed behind plastic trim and carpet sections.
It is most important to be sure not to foul seatbelt and roof mechanism. Wire needs to be well secured and taped down in those areas.
I used a Nextbase hard wire kit which was just the right length though I had to extend and then insulate the spade ends on the fuse tap as the shape/design of the fuse tap did not sit well with the MB fuse holder.
The inline 12 to 5v transformermabob is held inboard of the wee shoulder speaker and well away from the seatbelt with a strip of automotive trim tape and also wedged with a piece of foam for good measure.
Took me about four hours on Saturday past. That was going carefully and stopping to check how each piece of trim came off and triple checking that wires were away from roof mechanism and seatbelt. It also included the modification of the fuse tap where I cut the blades out of a spare fuse and soldered them to the blades on the fuse tap to double the original blade length. I also used shrink tube to cover what would have then become exposed blade.
Instructions for removal/refit of the various pieces of trim are easily found with a Google search.