Subwoofer (powered, under-seat style)
For many years, Subaru has offered an optional powered subwoofer amp/speaker combo that bolts underneath the front passenger's or driver's seat (black box on right side of this picture):
(picture posted by AaronBrown in this thread on WRXFanatics.com "04 stereo pin outs")
It measures about 10" x 8" x 2".
Here's a close-up view of the front side:
(picture posted by NewGTGuy in this thread on LegacyGT.com: "2005 Legacy Sub Installed Yesterday (Pictures)...")
The under-seat style subs come with a harness that you must install. Cars that didn't have the sub from the factory or port don't have the harness pre-installed.
The harness for this style subwoofer is a "T" -- it connects between the car's radio harness and the stock radio's 14-pin speaker/power connector. The radio gets all 14 wires like before, but the harness also
taps two of the speaker outputs (in parallel). It sends those outputs plus ground and +12volt power down the long leg of the "T" to the sub.
On most models, the sub is connected to the REAR speaker wires. However, some older sub harnesses connect to the FRONT speaker wires. Recently (~2008) Subaru switched back to tapping the fronts. They also changed to a new 20-pin radio harness on some models. See post #11, below.
On EARLIER model years (not sure which), the +12 volt power wire connects to the Accessory (key-switched) line. On LATER model years, it connects to the +12v Battery line (always-on). The factory subwoofer has an internal circuit that activates the amp when it detects sound on the left channel. The T-harness does NOT include an "amp remote" or "turn-on" wire!
The harness looks like this:
The pink tube (at left) is an inline fuseholder with 10 Amp fuse. The long leg, in the blue sheath, is about 50" to reach under the seat.
The connector on the subwoofer is a rectangular
8-position connector (same family as the radio's 14-pin connector) with only 6 wires used...
(picture posted by Packy in this thread on SubaruForester.com: "Is this connector under the driver's seat for the OEM sub?"
This subwoofer and T-harness will work with any 1995-up stock Subaru radio (except SVX). It will also work with any after-market head unit (that has speaker-level outputs), as long as you don't cut off the factory radio plug and do use a wiring adaptor (AKA a "forward" harness).
Simply connect the OEM sub's T-harness between the car's harness and your new HU, just like how it was with the stock radio
And yes, you can connect two OEM subs to put one under each of the front seats.
Just plug the two T-harnesses in a chain between the car's radio harness and the HU. Several people have done this with no problems.
The pins are numbered like this (viewing the connector on the subwoofer, or the wires exiting the back of the T-harness' subwoofer plug):
3 ### 2 1
8 7 6 5 4
The pin-out is as follows:
Pin Function OEM sub harness wire color
1 Right Rear + White/Brown
2 Left Rear + Violet/Brown
3 Ground (NC)
4 Right Rear - Black/Brown
5 Left Rear - Orange/Brown
6 +12V remote on (NC)
7 +12V constant Red/Brown
8 Ground Gray/Brown
NC = No connection — the factory harness has no wire at this pin position
Recent Legacies (and probably other models) have slightly different wire colors, but the pin-out has stayed the same.
Here's another post
that describes how the T-harness is wired (doesn't mention any factory colors).
There is more information about the internal workings of the subwoofer in this thread on ScoobyMods: OEM underseat Subwoofer info
Here's a link to Subaru's Subwoofer installation instructions (PDF files) for several recent models/years:
Subaru Technical Information System - Search the Online Reference Library
SubaruParts.com has this PDF for circa 2001 Imprezas
. (Later versions include an additional page that describes setting the two knobs. See subaru's Tech Info site).
The instructions vary slightly for different models/years, but the general idea is the same.
Here's a post
from someone who found a source for a replacement speaker.
The general consensus is that the OEM sub is just OK. It's not worth full retail price. It might be worthwhile, but only if you can find one used (with a complete, uncut T-harness).
OEM Kicker trunk and hatch subwoofers
Starting in 2011, Subaru offered OEM subwoofers made by Kicker.
(photos courtesy of cars101.com)
Their harness is a T, similar to the underseat sub, and taps the front speaker channels. Unlike the underseat subs, the T-harness includes high-pass filters for the outputs going to the main speakers. Also, the sub has no level or cutoff frequency adjustments.
See this thread for pictures and more information:
New 2011 11" Trunk Mounted Subwoofer
Aftermarket Under-seat Subs
Since the question comes up a lot, here's a post listing several aftermarket under-seat subs
that perform better than the OEM sub (and in some cases cost less).