Reddit mentions of Metra 95-8901 Double DIN Dash Kit for Subaru Legacy and Outback 2005-2009 (Black)

Oh goodie, so that's basically the easiest possible combination. (I was asking about steering wheel controls and subwoofers because you'd need even more adapters for those. And auto climate control makes the din kit a pain in the ass, you gotta import it from Japan. But yours looks like a base model without any of that.)

So you're going to need:

• A wiring harness (specific to your Outback). You want to connect the other end of this to the harness that comes in the head unit's box. Personally I used solder/heatshrink to connect these but "butt" crimp connectors are also an option. Please don't use electrical tape, that'll fall apart through time and heat. https://www.amazon.com/Metra-70-8901-93-Impreza-Vehicle/dp/B0002BEQL6/
  
  
• An antenna adapter (for FM/AM). https://www.amazon.com/Metra-40-SB10-2005-Up-Antenna-Adapter/dp/B000K53QAQ/
  
  
• A DIN kit: https://www.amazon.com/Metra-95-8901-Double-Outback-2005-2009/dp/B00K5D2MYE/
  
  
• The stereo itself: https://www.amazon.com/Sony-XAV-AX100-Android-Receiver-Bluetooth/dp/B01MF9W0GU/ or maybe https://www.amazon.com/Pioneer-AVH-1300NEX-Multimedia-Bluetooth-SiriusXM-Ready/dp/B073BNFTV7/
  
  Crutchfield also has some good resources. The only thing is that the aux port under the palm rest won't work anymore, unless you can find a way to adapt it to the new stereo. Many new stereos don't even support aux anymore. Also the Sony I mentioned (probably amongst others) has a USB port hanging off the back, so you'll need to get a USB extension and run it to somewhere in the car. Like drill a hole somewhere in the center console and run the cable up.
  
  And here's a guide for actually removing the stereo: https://www.youtube.com/watch?v=GIyQGdXUxS4