I'm doing exactly the same thing for a similar reason to you.
I want to keep the existing stock radio as its wired into my mobile phone and utilizes the steering wheel buttons
I've got all the bits and pieces and am now ready to start my project. I've going to take lots of pics and post as i go along.
Being a bit nervous about destroying the trim of my new beamer I searched on ebay for a center trim to modify. So now armed with this i can now experiment without ever taking a knife to my precious car. Check out here
So if any guys have put a screen in this position and can advise me that would be cool