8-pin Aux Retention Harness
In Subaru models listed at the top of this page, the factory HU has an 8-pin socket on the rear for its aux input. A factory harness connects from the back of the HU to the back of the 3.5mm aux jack (in the armrest center console in most models, or an "ashtray" pod in 2007 Impreza/WRX/STi and 2007–2008 Forester). That harness has proprietary plugs at both ends. The harness handles the stereo aux jack only, not the USB socket (if your car also has one).
The white 8-pin connector on my aux retention adapter fits the end of the car's aux harness that is (or was) plugged into the back of your factory-installed HU.
I'm currently offering this harness in either of two options for the aux connection:
Retain OEM "Odd-shaped" Subwoofer
Some 2009–2011 Foresters have the "odd-shaped" (non rectangular) factory-installed under seat subwoofer shown in this thread. In those cars, the OEM HU has a line-level subwoofer output on the aux harness. HU models with the 8-pin socket include PP243K6, PP343R6, and PP443L6 (and possibly others).
I can build my 8-pin aux harness with an additional male RCA plug and 8" (~200 mm) long amp remote turn-on lead. Prices: add $13 to the above configurations (3.5mm, plain wires, or RCA), or $20 for a harness with only the male subwoofer RCA and amp trigger lead (no aux plug or wires).
Note: The subwoofer RCA plug is only needed on cars which had the "odd-shaped" sub fitted by the factory. In cars that do not have that sub, the corresponding wires are not present in the factory harnesses (cars are not pre-wired for that sub). The accessory rectangular-shaped sub does not need (and cannot use) this RCA plug.
Retain Audio Steering Wheel Controls
Some Legacy/Liberty and Outback models use the same type of 8-pin harness plug for the HU's aux input and/or audio steering wheel controls. HU models include GX-204JE, GX-204JH, GX-204KH, GX-204LE, GX-204SE (and possibly others).
I can build my 8-pin aux harness with the two corresponding SWC wires. E-mail me the model information on your car and factory HU so we can discuss what you'll need.