In my '07 I just cut the plug out and left enough wire on the plug incase if I needed to ever put the stock one back in. Then I just wired it up to the aftermarket deck. I didn't see the point in buying the plug/harness when I can just cut and re-wire just as easy.
But if you do want the plug and installation kit go to Best Buy and ask someone for a Car Radio Installation plug for an '09 Aveo. That's what I did when I installed a friend's radio for her ford ranger and they just went in the installation shop and grabbed it from there stock pile of parts and handed me the kit.
you can only use the plug for 08 and up sedan, and 09 and up hatchback aveos... it is two smaller plugs, instead of one big one like on the earlier cars.
you will also need the onstar bypass thing, if you want to keep onstar.
ok man, see my '07 aveo already had the smaller deck kit installed because I purposely bought it with just an am/fm radio because I knew I was putting the one I had in. Made my life easier.