How discreet are you willing to go? Have you had a look at whether you can remove the plastic cover/trim behind your rear view mirror?
There is a chance you can fit a Mobius Maxi inside the cover, and only have the lens stick out. However, that can be quite a project to do properly and access to the buttons & SD card will be limited.
Otherwise, a wedge-shaped dashcam like the Viofo A119 V2 might be your next best option. I would mount it just beneath the sun-strip at the top of your windscreen, if you have one.
As far as cabling goes, the usual method is to run it into the headliner at the top of the windscreen (push it slightly in), across to the passenger side and then down along the A-pillar. Ideally, you should remove the A-pillar trim, make note of the airbags (if any) and run the cable parallel with the car's own cable harness down into the fuse box behind the glovebox. There are many ways to get power from the fuse-box from there.
If you are not comfortable removing trim, then you might be able to get away with running the cable down the weather strip, but it will be visible in multiple places.
Found a forum thread of a guy who put a Mobius in their C4 Picasso and has decent pictures to show placement and cabling. However, I have no idea if their model year is representative of the one you have:
http://c4owners.org/plugins/forum/forum_viewtopic.php?212740.post