The center console piece where the headunit goes comes straight off - it's got 5 or 6 plastic clips .. I took the cup holder out and just pulled the cover off (you don't need to worry about the HVAC controls - they come out with the console piece)
As for the doors, start from the bottom and pull straight out. Be careful not to pull at an angle so as to not break the plastic clips. There is a tiny overlap where the door cover and the rubber surround meet up at the bottom of the door
Check my previous post. Circuit City has the harness for the Subaru Impreza. Fits the STI just fine. It goes from Factory harness to their own "universal plug". You can cut the plug off and splice into your stereo, or they also have aftermarket specific harness that plug into the the universal harness for a quick install. Bring your stereo with you to make sure the plug will work.
As for taking the faceplate off: Pull out the drink holder, it takes a good tug. Then remove the drink holder tray. Two phillips screws then are exposed, remove them. Start at the top and work the trim loose. 5-6 plastic clips are holding it on. After you remove that, there's 6 screws holding the dual din mount in. I moved the plastic cubby to the upper spot and put the stereo on the bottom. It's easier to push the buttons when they are just a finger distance from the shifter. I have the factory speakers, can't help you there.