DaveG
Can't help you with WRX kit but check the tail end of this
thread as there are a few bits info on my Nokia install in there.
As for the line-in, I working on this too. At the moment it is not looking good as I don't like the idea of sending the signal via the aerial lead and I cannot find out how to trigger the many different cabled line-ins on unit (Aux in, TV in, CD changer in, Navigation in). Got the phone in working though, as above.
Basically I need to devise a Black box to send the right code down the control line to trigger the appropriate input. It is one thing to ba able to produce the signal but another to whork out what to say to it.