Sony car stereos have a "Source" button to select between the radio, CD player and any auxiliary equipment. It works in much the same way as the button labelled "Source" or "Input" on your television remote that allows you to switch between video game devices, DVD players and the like. As with the television set-up, the car stereo set-up requires certain elements to be in place before the "Source" button will work, including that the source be present and turned on.
Turn the source on if the Sony car stereo "Source" button isn't working. The source often is a secondary device in the car. For example, an MP3 player may be connected to a 3.5mm cord with a jack in the glove box. That MP3 player needs to be turned on and playing in order for the "Source" button in the "AUX" position to produce MP3 output.
Reset the Sony car stereo if the "Source" button continues not to select a source. Press the "Reset" button by detaching the front panel. Detach the panel by grasping the face plate and pressing the "Detach" button. It's the button with the U-shaped icon with an arrow in it. Look for the Reset button. It's often in the lower left side of the Sony car stereo. Press it with a ballpoint pen and try the "Source" button again.
Rotate the volume knob if you continue to have issues with the "Source" button not producing output. Try a source that's built in to the Sony car stereo, such as the FM radio. That way, you can eliminate wiring as an issue. Select the "FM" source and listen for a broadcast or static. Try a CD in the slot and listen for output. If FM radio and the CD produce output, the issue likely has to do with the wiring or the auxiliary equipment and not the "Source" button at all.