Your age and spec of car I'm surprised it doesn't already have an Aux in. I thought was standard on 3.0Si.
You have checked on the panel above the buttons for heated seats, run flat reset button etc to check its not already there?
I have factory fit aux in. Seemed to work fine when I had android phone but is kak for my iPhone. Have to turn both phone and stereo volume up max for crummy hissy low volume sound. Ended up getting intravee anyway.
Aux in comes very poor second to the intravee (Neatness, better sound, integration with steering wheel controls and screen). If you plan to keep the car for any length of time do yourself a favour and just go straight for the intravee.