AUX input addon to Audio system

Anything added must come from the dealer. GM figured out a lot of ways to separate you from your money after you bought the car. One of them is to build the Body Computer into the radio. That way when it fails, you have to get it repaired at one of their grossly over-priced repair centers rather than replace it with an aftermarket unit. A different radio will have to be programmed to your car. Also, if anyone in the past used a scanner to access the many computers on the car, they had the option to press the "lock" button. That ties all of the dozens of computers to the Body Computer. That programming can not be undone. If the radio / Body Computer has to be replaced, all of the computers on the car must be replaced. There is absolutely no reason to design a car like that except to cost you money.

That started with some 2002 models. Your car might not have the Body Computer in the radio yet but any replacement radio will still need to be programmed to the car. The scanner is needed to do that. Some independent shops have those scanners.

You can add the auxiliary interface or audio input jack to almost any Chevrolet vehicle from 1995 and up the body control module is not going to be a problem,
in order for the aux to work your Malibu radio needs to be XM satellite ready, most of the 2004 are not.
Because of that your only option is the rds kit or a hardwired FM kit
( not wireless )
search for p-14-fm-universal-aux-input-modulator-hardwired
good luck