I'm not sure if 6.5 inch speakers will fit in a Grand Prix. Unfortunately I know Bonneville's better. But anyways, this amp is what is powering my set up. I installed it myself in May. Hasn't given me any problems. Solid little thing.
Hitron HPA400.4 (hpa4004) 400W Max, Pro Series 4-Channel

However, if you want to keep your stock head unit you would need a Line Output converter to give the amp signal. That's how I have mine set up as well.