Some of the 2010-2011 OEM HUs have a "secret" line-level subwoofer output (on the 24-pin aux harness). The OEM accessory sub includes a T-harness that grabs that audio signal to feed the sub's built-in amp.
PE645U1 is one such HU model (Panasonic CQ-JF1860VD).
Some 2012 models continued using the same OEM HU. I don't think any 2013s still do. I'm fairly sure the CE617U1 doesn't have it.
For cars with the PE645U1 (and any other HUs with the sub line out), I can make a cable with an RCA jack and the proper pins to add to the empty slots on te factory aux harness. See this thread
on NASIOC (same situation exists on the Impreza/Forester Navi HU).
For cars that use the 20-pin harness for the radio power/speakers, I also make a harness
you can use to tap the speaker outputs more easily.
What is your HU's model number? [ADDED: Look below the "6" station preset button]