It's a straight forward procedure, Reberto, whether you have an original OEM or an aftermarket stereo installed. I chose to stay with the original radio - sounds spectacular enough to me.
The tricky part is modifying the Sub's wiring harness connectors. The SVX uses unique 20-pin connectors unlike and incompatible with other Subaru models, in which the same subwoofer is a simple plug & play. The sub harness carries all the same electric wires for front speaker signals, power, power antenna, and three three illumination wires, although in different pin positions.
Now, some folks have been in a hurry and just cut & splice but there is a member down in Texas that has SVX specific connectors that can eliminate such butchering. Well worth researching his posts and website for all the options. There's not any room behind the radio for a tangled blob of excess wiring because the back of the cage butts up against the HVAC ducting, so I chose to adapt the sub's harness to SVX plugs. Only adds a 10" bridge harness behind (which can be tucked inside the back of the cage or routed down below the radio. The cable to the sub underseat is a 50" long run down inside the console then down under the carpet at the seat's rear rail over to the sub.
There is a seat heater connector under the seat that gets in the way but like the driver seat, I removed the rear bolts and raised the seat rails 1/2" to get the clearance. Seats are awfully low in the SVX for a short 5'6" guy like me anyway...
David Carter has full information on pin assignments as well as a comprehensive How-To to extract the radio cage.
I carefully labeled the Sub harness wires, extracted the pins one by one and inserted them in the correct positions in the SVX male & female connectors.
Fussy but a piece of cake once you know how to cleanly remove the pins. Dave provided me with a cross section diagram of the connector catches which need only a tiny jeweler's screwdriver to release.
Dave's How-To has full pictures from installing an aftermarket radio that shows you everything you need to know. After I purchased the SVX connectors Dave emailed my more specific diagrams that made the pin adaption easy.
Dave's How-To:
http://ae64.com/SVXAudioInstall.htm
Dave's selection of SVX adapters for splicing installs.
http://ae64.com/SVXharness.htm
As I said, I bought just the connector ends and moved my own pins. Results is a tidy installation that is 100% reversible in about 20 minutes. You get really good at disconnecting the two RCA diversity antennas and removing the radio & cage to get at the backside.
cheapest OEM Forester sub from Byers Imports eBay store in Colombus Ohio; this is where I got mine:
http://www.ebay.com/itm/Subaru-OEM-2...item257d6e68f3