Also got the central locking happening from the drivers door key. Turns out the plastic appendage on the back of the door lock had developed a huge amount of play which is why it no longer locked the door so I fitted a different matching pair with a different key. I had salvaged these locks in 2016 and they were waiting for me in my box of stuff. Also there was a nice one with two keys for the Vachette. So we're all keyed up. Except the central locking doesn't lock the boot. Still have to pull the door liner on the rear passenger cause the window don't work.
Did you ever get the rear central locking working? I ask because my STI doesn't appear to have a boot actuator and I'm wondering if that's a model change between S1 and S2.