The holes are already in the back seats. You will have to cut the material & get the plastic lugs to push in. the problem is, the legs on the rear rests are closer toether than on the front rests. So unfortunately you will need to find rear specific rests I'm afraid.
You can feel the holes in the seat's. If you run your finger over the top and all the way along you will feel 6 dimples that's where you cut. The kit's used to be £40.00 each like michaelperrin plush the mountings. They are now £56+VAT including the mounting brackets but you would need three kits for all the holes but a word of warning my MK3 200 had three and the rear vision was very poor but my SW has two but only has two seats in the back and the rear view is a lot better. You may get the rest's from a scrappy but the mountings will not come out in one piece that I can guarantee.
At the last time I looked the ZR and SW ones wear only £5.00 more. Some of the MK3 200 ones are been sold off cheaply the Puma and Ash Grey ones wear sold for £20.00 each. Lots of the old 200/400/100/Metro/Mini and 600/800 range wear selling cheaply to shift stock of what X-Part said is 'Dead stock'.