Hi jacka,
I did this a while back. The extra wires in that plug run up to the splits. As for which colour goes where:
2003 Subaru Legacy Car Stereo Radio Wiring Diagram
Radio Constant 12V+ Wire: Blue/Red
Radio Ignition Switched 12V+ Wire: Yellow/Green
Radio Ground Wire: Black
Radio Illumination Dimmer Wire: Purple
Radio Antenna Trigger Wire: N/A
Radio Amplifier Trigger Wire: Green
Left Front Speaker Wire (+): Brown/White
Left Front Speaker Wire (-): Green
Right Front Speaker Wire (+): Red/Yellow
Right Front Speaker Wire (-): White/Black
Left Rear Speaker Wire (+): White/Red
Left Rear Speaker Wire (-): Red/Black
Right Rear Speaker Wire (+): Blue/Yellow
Right Rear Speaker Wire (-): Red/White
I used spacers from eBay to ensure the speakers did not interfere with the window mechanism. The holes in the ring line up perfectly with the existing holes. It ships as a left and a right side.
If you want pictures, I can probably find some.