Firstly, thanks to @Twigman for posting this http://www.gt86.org.uk/forums/topic/3364-folding-rear-seats-down/ and inspiring me to make life easier when I'm putting in my golf clubs. Also @Brig for the critical info.
For this mod I had to order a suede effect 3mm thong and a leather thong. The suede effect was for the mod, the leather one was just for wearing whilst doing the mod. I also ordered a couple of fabric Subaru key rings, given the size of these when they arrived, I have no idea who would use them as an actual key ring.
I also grabbed my long bendy grabber thing and trim removal tools for popping clips. Please excuse the mess in the boot, I should have hoovered before instead of after.
I started by laying the seats flat (hopefully for the last time via the buttons) and removed these two press clips. I don't think it was strictly necessary to undo the left hand one, but it gave a big more room to work. The blue shaded area is approximately where there is a square cut out of the backseat, behind the fabric, I used this later.
Most of the effort is focused around this hole in the fabric where the backseat mechanism latches, it's just below the button and impossible to miss. Fold this back a touch to get a view in there.
Close up.
I lowered a doubled length of cord down into the hole and used by grabber to grab it and pull it back up the other side of the metal rod. You can check which is the right rod by operating the push button in the seat, the rod will move slightly. Once I had the cord around the bar, I looped the other end through and pulled it tight. This material seems to have the right amount of friction to get purchase on the rod. Dropping the cord in and then grabbing it from the other side is a PITA and easily the most infuriating step, it took a few attempts and some coarse language.
I did initially just leave the cord coming through the factory fabric hole but I found that up there it interfered with the latch when the seat was locked into position.
A better solution is this is the lower hidden hole, which is behind where I marked in blue in image three, I used the grabber to pull the thong through here.
I then took a scalpel and made a very small incision in the fabric and threaded the cord through. I tested it a few times, one I confirmed it was good I fitted the Subaru key rings as a finishing touch and to avoid the cord getting pulled back through the hole. Pop the clips back to complete.
Do this on both sides. All done.