I read on another forum somebody was asking about dash cams. Somebody mentioned Halfords and then an Halfords member of staff advised against letting Halfords work on there car. They said that there is not training, they are basically put in the job, given some tools and they have to just try and do what they can. It would be the same as you trying.
If you can get from the top of the windscreen to the fuse box (which I think is in the engine bay) via the trim, you can get a piggy back fuse. You unplug a fuse, plug the piggy back in and the put the fuse in that, then all you need to do is wire the earth to a screw on the body work. You would need to see where Mercedes have run the cables from the engine bay to the dash.
For the rear, if you are lucky enough to have a 12v socket in the boot then you could just plug in there, otherwise if you are like me without one, then you would have to run a 12v cable from the front.
This kit is for the Nextbase dash cam, but you should be able to buy the parts for all. -
https://www.ebay.co.uk/itm/Nextbase...rd-Wire-Kit-Car-Dash-Cam-Camera-112-212-312-402G-412-512-512GW-DUO/291517377396
I used to put in alarms and car stereo, amps and all sorts on my cars and my mates. The bad part is getting the trim off without breaking clips.
It is surprising that with most cars having rear camera's and the augmented reality having a camera at the front, that Mercedes didn't have dashcam recorded fitted in, it would be another option and a safety feature.