In cars without the factory harman/kardon or McIntosh systems, the harness for the OEM underseat sub is not pre-installed. The harness will only be there if the powered subwoofer accessory kit was added at some point (by the port, dealer, or previous owner).
Sedans with the h/k system have a sub on the deck behind the rear seats. The wires run to the h/k amp (under the passenger seat).
For your non-h/k Legacy, the best/easiest way is to access the speaker outputs behind the HU. I have harnesses that make that much easier. See link I posted above.