My mid 2016 Model S came with a rear floor mat that has velcro patches on the underside, but no matching velcro on the carpet like the front has. The mat slides around and bunches up so I decided to fix it: I bought a set of new BMW twist lock velcro adapter pads off of ebay for about $10. They require the retainer that is mounted to the BMW carpet so I 3D printed some custom retainers. The mat has several velcro spots, I'm using the three closest to the rear seat. The BMW retainer fits the velcro patch nicely. To install the retainer I attached the retainer to the mat and then pressed it lightly into the carpet to leave a dimple where the retainer needs to be mounted. I then used a very sharp razor cutter to make a ~1/2" (12mm) cut in the carpet. Next, remove the velcro retainer pad from floormat, press the retainer stem through the hole and then slide the 3D printed retainer under the carpet and lock it in place. You can access the underside of the carpet by pulling up the back seat bottom and moving it out of the way. With the retainer locked in place, you can easily position and remove the rear mat without popping the velcro retainer pad out of the carpet. Other tips: It's possible to access the underside of the carpet by removing the threshold trim piece. Removing the lower rear seat requires a FIRM pull at the retainer location on either side of the seat. There is also a chance you can break the plastic insert but they are available from Tesla if you do damage one. To pop the seat back into the retainers you may have to sit on the edge of the seat and bounce on it a bit. You will hear/feel it pop in. Here is the .stl file to print your own retainer plates to hold the velcro pad in place: http://jelloslug.com/Tesla/retainer plate.STL You can buy retainers that look like they will work with the velcro retainer pad stem, but they looked a bit small and flimsy to me.