@DrValkyer
Did you grab that circuit from the motherboard?
It's from A500 keyboard... In fact, the metal part of the keyboard and the keyboard matrix (the thin green thingie inbetween layers) is from A500 (that is, only the keys themselves are from A1200).
This way the the only thing I had to do was to solder a connector to A1200 (with wires going directly to the mobo, and disabling the onboard keyboard controller).
Full instructions are available from aminet hard/hack (somewhere, can't remember the file name right now).