Here are some empeg/WRX install pics. You can email me with questions as well. I did mine and helped my buddy put one in: http://www.persiconi.com/wrx/images/audioinstall/install.html

You can keep the Subaru wiring, but the speakers are made of paper and have TINY magnets. The speaker install on my page is overly complex. My buddy did a better job of ordering some speakers that fit with less modifying. I can contact him for model numbers.

The Subaru headunit is a dual DIN 6 CD changer. Pretty nice I guess. But what's really good is that you can replace it with an empeg AND a headunit. So the head unit can power your speakers and you can reuse the factory wiring. Jim Hogan went this route.

My page is for a 2002 WRX. My buddy's was a 2003 WRX. The 2005 WRX has the same guts (as does the 9-2x).

The upgraded speaker options and subwoofer options are worthless on the WRX. The money you'd spend on it could better be spent on some really nice aftermarket speakers.

EDIT: I think my page would apply to an Impreza based car. For example: Forester, Baja, Impreza RS, 9-2x, Impreza WRX STi, Impreza Outback. The Legacy is where you start running into issues.