what do i need to install a aftermarket stereo on 2002 grand prix

I suggest you talk to the folks at Crutchfield.

There is a kit you can buy that allows you to keep the steering wheel controls. I agree you need to consult the pros to find out for sure what all you need. In fact I would consider having the whole thing installed by a professional. Modern automotive electrical systems are so complex that altering them in any way can create all kinds of new problems that will cause you to hate your car.

You will need to buy a stereo that's compatible with your Grand Prix and the installation kit to go with it. This will include the brackets for the dashboard and a jumper harness to plug your new stereo into the vehicle wiring harness. DO NOT under any circumstances cut the vehicle wiring harness!! If the jumper harness won't plug in, you have the wrong one and it needs to be exchanged!! When I bought my 2000 Grand Prix it had a Pioneer stereo in it. I wanted to return the car to its original stereo, which was still with the car. The Pioneer worked fine, including the steering wheel controls, but I wanted the car to be original. I simply unbolted the Pioneer from the dash, unplugged the jumper harness and antenna connection, and put the original stereo back in. The whole thing took about 20 minutes. So, what I'm getting at is you can replace the factory stereo and it will still work correctly and not mess up the rest of the electronics in the car if you get the correct installation kit and follow the instructions. Hope that helps! Jim