So what would you have done differently? Bigger sub, right? Told you to go w/ a 12. Or at least a 10.No...seeing as how I just did this. There's lots of work to add the sub and the 5th and 6th speaker to the standard sound system. I just redid my monsoon system and replaced it all, but used the monsoon sub box. So here's what you would need to do. You'll need to find an amp. Toroaudio.com makes some nice, cheap ($100), small amps and I would use these if I were to do it all over again. You'll probably need a new headunit as well to get the bass. You'll need a monsoon enclosure unless you're making your own, because the room behind the panel is scant at best and GM did a nice job utilizing the space. (Trust me, buy the OE and enclose it in fiberglass...it'll save you some big headaches). Once you get the box, in order to get a decent sound on an aftermarket subwoofer, you'll need to cover the box in fiberglass.
I know, I know....it sounds like a lot and you probably don't believe anything I just said. I was that way 2 months ago when I first started doing my complete system in my car. I was a complete newbie at car stereo and I can tell you where I made my mistakes and what I would have done differently. I'll save you tons of time and money on it if you need help and advice. Please don't do what I did and not listen to everyone else thats already done it....LOL I could have saved about $1k on my install and about 3 days of time.
EDIT: If you wait till the spring, I might even have a decent fiberglass mock up....depends on my ambition and employment...LOL