Radio install question

[Noob]

what is the harness called that they need to install when putting in an aftermarket head unit. the sales guy said it changed on price depending if i had onstar and things like that. what is that part called ? im dam sure not paying what they want so i need to buy parts myself.

I have an 03 bare bones with no bells and whistles if that helps

 

[NathanJax]

Go here...

http://www.metraonline.com/products

and it will tell you what all you need, including which radio harness