There is one significant wiring difference between WRX and STI models. Four jumpers are fitted to the ECUs bottom board to allow configuration.
Fit the jumpers according to the diagram below. DO NOT fit the jumpers in any combination other than the two shown.
Depending on the country the vehicle was destined for the labelling of these jumpers (WRX and STi) may be incorrect. It may be necessary to try both options to determine which one is correct. If your vehicle is an STi and the ECU powers up with jumpers in the WRX position don't be alarmed.
If jumpers are in the wrong position, the ECU will not power up.