The WRX harness is basically one big harness, not a seperate bulkhead, front, back etc. like the RS's. You're going to be doing a lot of cutting with the wrx harness to get to just what you need.
http://www.sportcompactcarweb.com/tech/0403scc_subaru_gbd_wrx_engine/photo_01.html
^Go through those pictures and read the captions, plus read the corrisponding story to it to get an overview on what needs to be done. Do NOT follow that article word for word because it is said to be inaccurate in some places. But again, its good for getting a little overview at what you should be doing.
I'm not trying to judge you anything, and I respect you for taking on a big project like this, but just from what I've heard it sounds like you need to do a little more research. Check out some of Brydon's threads, especially his sticky at the top of this forum. Also check out the "Good Wiring Help" sticky and read around