I had to order the harness for my 2000 Alero from a speed shop normally you just cant go out to walmart and get the right harness for a Alero trust me I tried 2 walmarts and I also had to get the mounting kit from a speed shop and it wasnt even marked as a Alero kit but a Grandam kit but it still worked the same. There is also a Harness that you can get that costs about $80.00 Dollars I dont know what it does but I may replace the chimes when they showed me that harness I told them that it couldnt be right. And also when you put everything back together you may have a pisser of a time getting the head unit to fit without moving around the wiring harness to you can slide it in, atleast I had a problem with the harness getting in the way. Since I guess you a new at installing radio's I would have bought it from
www.crutchfield.com all there head units and everything else they sell comes free with the right harness and mounting kits for free.