’00 outback wiring, speaker ideas?

Okay, I'm looking for some advice... I just bought a 2000 Subaru Outback Limited. It has the upgraded audio system - two pairs of 6.5s, two pairs of tweeters and the under-seat sub, but of course I'm upgrading most everything. I want to put a real sub in there but I don't want to sacrifice the wagon area. I'd appreciate your suggestions. I'm thinking about a custom box on the side (like this one, http://cushman.net/category/projects/empeg/) - maybe both sides, but I'm concerned I'll be limited to a 10". Anyone seen any installations in a wagon that don't use up all the wagon or spare tire area? I've only ever had a Kicker Solo Baric 12" in the trunk, but I want something with better sound quality. I'm currently powering the sub with a Rockford Fosgate two-channel amp - bridged into 250W RMS (underrated). Anyway, I could also try to build something from fiberglass to put under the front seat. Has anyone tried those Pioneer shallow-mount subwoofers? Are there other brands of shallow subs you like? I know it's a compromise, but how much of one? For the front, I've got some leftover CDT components that I plan to put in the stock location in the door, which will require adding a tweeter in the door. They're powered by a very old Soundstream amp that has 50W per channel RMS. There's a second set of tweeters that usually go in the corner pillars that lift the sound stage higher. However, the stock system has tweeters in the corners of the windows - the window sail I think it's called? Any opinions on whether the sail or the pillar would be preferable - or whether it would be easy/possible to replace the stock tweeter? Finally, I'm looking for a stereo wiring diagram. The12volt doesn't seem to have it. Any help out there? All suggestions are appreciated.