Verified answer

If none of the factory speakers are blown (buzzing, etc), then I suggest starting with a new headunit. This will give you superior sound quality versus the factory headunit, give you some sound shaping options to tailor to your tastes, as well as a slew of other convenience features (bluetooth, iPod connection, USB, etc).

Unfortunately, late model General Motors vehicles run on a LAN system. The headunit is integrated with some other things in the truck. So to retain all the other things in the car when removing the factory headunit, you'll need an adapter module. It's the Metra GMOS-01. A little pricey at ~$63 on Amazon, but works perfectly. You'll also need an antenna adapter (40-GM10 $7) and dash kit (99-2003 $10).

As a separate note, you may want to re-think your stance on a subwoofer. A subwoofer is an integral part of a well rounded audio system. They're not just for teenage kids. Movie theater use them, every concert of every type of music uses them, and every home theater system includes one. You don't need to go crazy. One small 10" sub should provide all the low-end you need. Additionally, letting the subwoofer handle those low frequencies will allow you to remove the low frequencies from the other speakers and clean them up a bit. Heck even the Kicker hideaway would likely give you all the bass you'd want and can mount in nearly any location without taking up space:

http://www.kicker.com/Hideaway

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