I have just been through the same problem with my BP, turned into bit of a nightmare. I originally just wanted to plug in an Ipod to the factory setup.. and bought a Harmon Kardon drive and play, waste of time and as there is no aux input in the standard unit, the FM modulator was the only option, and that was very average. I ended up bypassing the whole system and mounting an ipod only headunit in the top vent pocket, and it works great.
The double din conversion panels are very expensive and this is by far the cheapest option, from AVOJDM the genuine Subaru kits for this end up costing about $160 shipped ex Japan. They say 04+ but im sure the 03 is the same fitment.
http://www.avojdm.com/product_info.php?cPath=26_54_60&products_id=115
and as you have the non nav model with the standard pocket size (unlike me, as I have to fill in a gap around edges) it will be a direct swap and look almost factory.
This way you can easily convert back to factory and take the after market headunit with you when you sell the car.
Edit: just realised this original post was from 09', oh well might help someone else now.