Today I'm installing Sirius Satellite Radio to the stock stereo system. In the future I plan on upgrading to a better sound system but until then I've been paying for my Sirius on a car I don't own anymore. I transferred my account to this one over the phone in about 5 minutes and I'm glad to have my satellite radio back.
The install took about an hour with taking pictures and I give it a 3 out of 10 for install difficulty.
Check out my build thread (HERE) to get more installs and daily updates on this car.
I bought the kit from Subaru Parts Mall found here. The price was $305 once I added some other products, the more you buy the bigger the discount gets. It's the OEM parts you would have received if you went straight to the dealer except it's not $500
Here's the install:
Stock radio... Remove the radio surround by slightly pulling around the sides and then work from the bottom towards the top. It should easily come off without any tools needed.

Remove the stock radio by unscrewing the 4 Philips screws on the outside 4 corners. Pull out slowly until you have enough room to unplug all the harnesses in the back of the radio.

These are all the parts that come with the kit.

Put the side brackets on the tuner, then the power harness, lastly the antenna adapter.

Install the satellite tuner above where the stock radio was with two Philips screws.

Remove the drivers side A-Pillar. To remove the pillar, take off the SRS cap with a small flat screwdriver or utility blade. Then remove the 10mm bolt behind the SRS cover. Slightly pull the pillar from the top down to the bottom. Reveal the green Antenna plug towards the bottom.

Prepare the new antenna with adding three pieces of foam (supplied) to reduce vibrations.

Mount the new antenna on the top right of the windshield (inside), secure it to the windshield using the supplied double sided tape. Next run the antenna cable across the top of the window tucking it under the headliner. The pink connector at the end of the wire should reach the top of the A-pillar.

Use the antenna extension to go from the pink plug at the top of the A-Pillar to the bottom green stock connector. Use the supplied foam to cover both connection to prevent rubbing against the plastic a-pillar cover.
The top right picture is the other end of the stock harness that is located under the stock radio. I removed the center heating duct (2 screws) to locate the plug easier. This end will plug into the small antenna wire that is coming out of the tuner.

Once you secure every thing back up, you can plug the radio back in and power it up. You must call to have your subscription added to this radio. Once the signal is beamed to your car, the radio will begin updating to your stations.

When your radio is all programmed and playing your satellite stations, then screw the radio back in, put the trim cover back on, install the A-pillar cover, and enjoy!!
Thanks for reading!