Yes, the STi is wired for sound. Crutchfield has a mating connector for the head unit so there's no cutting wires involved. The bracket that the cover plate is screwed to has holes to fit a standard DIN head unit. Wiring is in the doors for the speakers too. The only thing they left out is the plastic nut plates that the speaker screws go into in the door panels. The Subaru part number for those is 84965AA060 (Screw Grommet). You need 5 per front door and 3 for each rear door.
slashSTI, the holes in the door are square about 1/4 of an inch. The grommet will fill this hole and allow you to use a screw. I didn't have these so I got some nuts bolts from HD and just used some nylon washers to hold the speakers to the door. No big deal since you have to use adapters anyway. Use the grommets if you can as it it very hard to get your hand behind the door panel where the tweeters go. I would think you can get them from a dealer.
TweetyBird, there are many many threads about this topic on the internet, on this site and several others. Try doing a little searching before asking very general questions. That's why you got little response.