16-pin Aux Retention
In Subaru models listed at the top of this page, the factory HU has a 16-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). That harness has proprietary plugs at both ends. The harness handles the stereo aux jack only, not the USB socket.
The white 16-pin connector on my aux adapter fits the end of the factory 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 2011–2013 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 sub out on the 16-pin socket include CP635U1, CP435L1, and CP235K1 (and possibly others) — HUs with 4.3" display.
I can build my 16-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 $22 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 Foresters 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 Car's Additional Signals
Non-nav 2012–2014 Impreza (not WRX or STi) and Crosstrek models have additional signals on the factory 16-pin harness: Parking Brake and Reverse Gear. 2011–2013 Forester models with HUs with 4.3" display also have those signals. No other models/years have these signals on the factory 16-pin aux harness.
Non-nav 2014–2015 Forester and 2015 WRX/STi with the OEM harman/kardon amplifier and speaker system have the h/k amp remote trigger signal on the 16-pin aux harness.
I can build my 16-pin aux harness with corresponding wires for any of those signals . E-mail me your car and factory HU information so we can discuss what you'll need.