I've never pulled the rear seat from a W114/W115 but I imagine it's exactly the same. It's the same between W116's and W123's. First you pull out the seat squab (using the two release buttons on the bottom front) and then there are 3 bolts to remove to free the back - one on either side of the bottom of the seatback, and one centered under the armrest. Then the seatback lifts upward to unclip and pulls right out. It's not a hard job at all and if you have a place to sleep, you have plenty of room for it. I suspect the most difficult thing will be getting the holes aligned properly - you don't want to make them in the wrong place and ruin your skin! Once the hardware is in place, you can feel where the receptacles are and poke small holes just enough for the headrest posts to fit in. Once you have new plastic trim pieces, you can probably find a measurement of exactly how far from the back they're supposed to be - other than that I think they are simply centered within certain pleats just like the front ones.