Remove the piece of console trim that holds the lighter socket, and tighten the retaining ring on the back side?
On my 2001, that means you pull up the shift bezel (by hand, no screws), remove the ashtray, unscrew two hidden screws from below with an offset bit driver, and then pull off the radio/ac trim by hand.
Check the factory manual or a stereo install manual for details of your model. Or ask your local subaru dealer or stereo installer. Or maybe someone else with a 3rd gen outback will chime in.
btw...if you want to wire an iphone adapter into your stereo, now is a good time.