Try a company called peripheral electronics, I put one of these in my 2003 silverado. It plugged right in to the back of the factory stereo, the hook the ipod into it. It sounds great since it is a direct feed from ipod to stereo.
Here is the web site
http://peripheralelectronics.com/2002/aux_in.asp
I believe this is the one you would want
PXGM9 1996-2002 GM Vehicles
. Connects to 9 pin auxiliary input port on the back of the factory radio. Provides two pairs of stereo inputs.
Note: Requires factory auxiliary source such as; console or dash mounted cassette, CD player or console mounted CD changer. QUICK REFERENCE
• Provides TWO sets of Stereo RCA inputs
• Requires OE add-on unit such as slave cassette player, CD player, or CD changer