I've confrmed that the '08 Impreza aux-in kit does work since the pins are the same size. You need to release the pins from the white connector on the Aux-in harness, and move them over to the I-85 connector. On an '05 Legacy or outback, pins 1, 2, and 5 are empty on the I-85 connector, so you will only need to remove pin 6 from the I-85 connector so that you can install the pin from the aux in harness
Pin 1 - audio left (white wire)
Pin 2 - audio right (red wire)
Pin 5 - Ground (black wire)
Pin 6 - Aux enable (brown wire)
Those are the colours of the wires in the Aux-in kit harness, and the corresponding pins that you need to move them to on the I-85 connector. This allows you to have aux-in and still retain the steering wheel audio controls