I recently bought an Infinity Basslink for my WRX sedan. It sounds great in combination with the stock HU and stock speakers, is cheap (about $200 + shipping on Ebay) for an all-in-one combo of enclosure, sub, and amp, and it doesn't take up alot of trunk space.
It provides more than enough bass for me and I like the way it sounds better than Bazooka tubes.
Mine is mounted vertically in the left rear corner of the trunk with the connections facing the cabin and the knobs facing outward. There is enough room behind it to get to the jack, it doesn't interfere with the spare tire well, and the brackets can mount to the metal using self-tapping screws.
While it can be connected several different ways, mine is connected behind the head unit using the included speaker wires, as opposed to getting a line out converter and using RCAs. The speaker wires were run down the passenger side and the power cable down the driver's side.