Rapidly pushing the audio source button on the steering wheel will get you where you want to be in the audio system faster than pushing once and waiting for the system to change between one source and the next before pressing the button again.
For example, pressing the source button rapidly 5 times will get you from FM1 to AM. That is, assuming you have no other audio sources connected other than the over the air ones. For example when I have a bluetooth audio connection to the car (iPhone) I need to press 6 times rapidly to get from FM1 to AM. It could be with a CD in the drive I would have to press 7 times, etc.... Give it a try! It's not direct, but the changes from one source to the next are pretty much instantaneous.