2005 malibu, amplifier

You need to find a switched wire under the dash somewhere that is independent of the radio.  There is no switched wire behind the radio because of a digital bus that turns the factory radio on and off.  In newer chevy's, especially the trucks, there is a large white wire under the sterring column which is ignition switched.  but Don't take my word for it, use a multimeter to check it first. There is no such thing as free installation!