Thanks for the help. I already had a fascia and cable adapter kit, but didn't realize the piece of plastic was for the rear of the stereo, makes sense now. I bought a cage this afternoon and think I have everything fitted and working.
One question though, is the cage supposed to be fixed to the hole in the dash/the new fascia in some way? as I have it, it can't be pushed into the dashboard (because of the rear bracket and the cage overlapping the new fascia at the front), but it can be fairly easily pulled out towards you. Is this normal?