2005 Chevrolet Cavalier Deck Install?

I have an aftermarket deck that I would like to install in my cavalier. In the past I had wired it to my truck and all I needed was a $20 wiring harness and dash kit. I went to best buy to get the parts to do my cavalier and they are saying I need to spend $125 on a special onstar harness. My car is very basic and the radio isn't equipped for onstar let alone a cd player. Do I really need this harness? Or is there a basic harness I can get at walmart?