Okay so, here's your problem. You can't install the Subaru Factory underseat subwoofer because it isn't made for your model year. You can only install the trunk subwoofer if you wanted a direct hookup for Subaru Equipment.
It definitely isn't worth
it to buy the factory subaru underseat sub and then try
to make it work, the cost per gains simply isn't there. You're better off buying an aftermarket option like the Pioneer Slim loaded sub box with an amp:
or the Sound Ordnance B-8PT: