Okay, I will chime in for a minute here.

You can use your stock stereo, with proper speakers and a bazooka tube, you can make your stereo sound pretty okay.
I mean, it won't win any sound competitions, but it will do it's job. I have used a bazooka tube, and aftermarket speakers on my 08 Aveo LT, and was quite pleased with it.
If you don't want to lose that much trunk space, you can go with something different, like a basslink, or the Blaupunkt powered sub. They all work pretty well if you are looking to add just a little more depth, but not wanting to go all out, and like the tube do not require a head unit direct connection. It requires a single input wye'd off your rear speaker, it doesn't require a remote lead, as it will auto-on when a signal is detected.
On the bright side, of you buy it, and find you don't like it, you can always send it back, quoting, "This thing is junk"