Well the dealer went ahead and got the fm modulator put in and it didn't cost me anything, so that'll work for now. I've found a
device that may do the trick though. It's called the PAC Auxiliary Audio Input Interface GM12. It plugs in the stereo through the XM input, and the XM plugs into it. Then it has a three position switch that allows you to choose between XM and two different 1/8" inputs on the device. I haven't verified that it will work in the Grand Prix yet, but once I can confirm that, I think I'll order one and install it myself.