I don't have any specific experience with your model, but I've added an Aux input into my last 3 Subarus with stock radios, in 3 different ways.
Look up your specific radio (either with a part number/manufacturer on the faceplate, or use your VIN on parts.subaru.com and look up the subaru number there, the former will likely be more helpful, though). Subaru doesn't make their own radios, and they use many different manufacturers, so there will be different answers depending what radio you have.
Every Subaru radio I've looked at in the last 25 years, has had an unused CD changer plug on the back. The McIntosh in our '04 Outback VDC, I bought a yatour digital music adapter, which plugged right in, and was super simple, but cost like $100.
I had a '97 Legacy that had a pretty simple input, I was able to make a quick adapter using a rocker switch and a panel mount audio jack.
When I looked into it on my '00 Outback, I don't think the Yatour one was available (or I just didn't want to spend $100 on a $500 car), and when I looked into a DIY, I discovered it was a digital signal, so would require a converter. I found someone who made a converter for it designed to integrate into an ipod or thumb drive, but was way overkill for what I wanted.
I ended up just using a hardwired signal modulator. Works the same way as one of those crappy remote modulators, but it's actually wired inline with your antenna, so it gets a much better connection to the radio, and disconnects the car antenna so you don't get any interference. I got this one, and their install kit, simple install, works great.