In case you're still working on this (or for anyone else who comes along later)...
Have you seen this?
2003 Forester CD Changer installation guide (PDF file on SubaruParts.com. Shows how to open the dash and remove the factory radio).
This thread I put together on NASIOC may be helpful, too:
Stock head unit information -- pin-outs, subwoofer, aux inputs, and more
And, of course, there's always
www.subaruforester.com 