A car CD player many not function due to a mechanical or electronic malfunction in the player itself or due to problems with the electrical system of the vehicle. Determining if only the CD functionality of the system is impaired can help narrow down the cause of the issue.
A malfunctioning car CD player that can still play radio stations normally is likely to have a problem with the CD player mechanism itself, such as a dirty optical pickup or defective spindle motor. A CD player that can power up and play CDs but produces no sound may be improperly wired into the speakers in the car. A CD player that cannot power up at all may be internally defective, or may not be receiving power due to a blown fuse or other wiring fault in the car's electrical system.