Even though this thread is old i'm still gonna post.
They make aftermarket wiring kits for the GM vehicles of this sort (Impala, Malibu, etc.)
These wiring kits are not cheap, 60 bucks or so, but they have a digital interface thing that keeps your chimes and everything and you do not have to keep the stock reciever in the car. I just installed one on my Malibu and it works great.
It also simulates the Ignition wire in your car since mine and maybe other GM cars didn't have one behind the head unit area.