How do I fix the cruise control in a Chevy Malibu?

It would help to know the year of the car you are working on as there are different cruise controls for different years. I am assuming it is a late model as per the buttons and light on the dash. The first thing to do is check under the hood to be sure everything is hooked up mechanically and that the cable to the throttle body is okay. If that is all okay you need to use a test light to determine which circuit is faulty. You will also need a copy of the wiring diagram so as to see which circuit uses which connector and the color coded wiring used. Then simply touch every connection to the cruise module that is on the other end of the cable under the hood. The cruise needs inputs of battery power, ignition power, ground, cruise on, set, accel., resume, VSS, brake, and possibly more. The module only cares about an input of 12V or a ground supply. The VSS is a pulse voltage which rarely is faulty, but it is to do with how fast the car is going. Generally if the speedo works, this is okay. You have to check all these inputs one by one by disconnecting the electrical connector at the module. Then hook your 12V test light to ground and go through all the switch inputs with the ignition on. The light on the test light should light while you use the different switches for the cruise. To be sure you have a ground, hook the test light to power and check the ground side. If everything is okay, the module or cable is no good. If you have an input problem, chase this back for either a broken wire or an open switch. Good luck.

Take to the dealer!