Try looking up OEM part #86257SA010. This is what came stock in my 2008 SG forester. I just checked the subaru parts website and it looks like they didn't start offering this option until 2007.
FWIW its probably cheaper, easier and more beneficial in the long run IMO to just go with an aftermarket head unit with bluetooth/aux/USB etc. They aren't very expensive and are pretty straightforward to install. That's the route I went with mine, just be aware the the OEM subwoofer if you have one will be disabled. I went with a single DIN pioneer unit and added a 150W powered-subwoofer under the passenger seat which works great.
That being said, I have my old OEM 6 disc changer and aux port up for sale on OfferUp, you may be able to retrofit the aux port in an older model but I'm not certain.
