The Monsoon was an amplifier setup for GM cars. It is highly integrated through the BCM and I don't think it'd be worth your time to try.
You do have options if you absolutely gotta keep the OEM radio. For higher quality sound, I would second the addition of an amplifier. You'll need a line out converter like this one to send a signal to your amp. This is the amplifier I used (I have a Pioneer navigation head unit). Of course, your stock speakers won't be happy, so you'll need to look into replacements. And if sound quality is important, you've gotta have a sub. This is the one I'm pretty happy with. You'll need yet another line out converter to hook it up.
This will allow you to use an iPod as an aux. There are other adapters that will give you a 3.5mm jack to plug other audio devices in. There is indeed a six disc changer available, I don't know if it'll work in your non Monsoon setup or not. It will need to be programmed, either by a dealer or a buddy with access to a Tech-2.
