I just installed the blackvue 650 2 ch in the S. It was a breeze. I had done it on our Audi Q5 and that was a bit of a pain because the Audi fit and finish was better, which meant everything was tighter and harder to squeeze the cable between the pillars and headliners for the rear cam.
The model s was super easy. And what was nice was I had the AC going so I was also super comfortable. Took about 15 minutes to install both cams.