Originally Posted by cef2lion
From a totaled 2008 Forester I picked up a C123 HU for my 2005 Forester. ...
[aux doesn't work]
Correct -- pins 4 and 8 are the amplified (speaker level) subwoofer output. The 3rd wire you cut needs to be connected to chassis ground. The aux pod has a switch that will connect that to the HU's aux enable (pin 6), which will then let you select the aux input.
For anyone else installing a C-123 (or C-124) without the factory aux jack, you can simply ground the HU's pin 6 to permanently enable aux.