The most effective and safe way of removing a CD stuck in a car stereo is going to an automotive professional. There are some methods that can be done without professional help, but they carry a degree of risk. Some car stereos have a force eject feature, enabled by pressing the car stereo's power and eject buttons simultaneously. Every car manufacturer has different combinations for the force effect command, which is outlined in the user manual.
A more direct method of removing a CD is with a knife and tape. Put tape over the end of the knife with the sticky side facing out, and cover one side with paper. Use the sticky side of the knife to remove the stuck CD, being careful not to damage any of the internal components of the stereo. To be safe, this method can also be done with an object made from a non-conductive material such as a popsicle stick or pencil.
Another method is giving the stuck CD traction against the eject mechanism with a blank CD. Place the blank CD about halfway into the slot, above the stuck CD. Press the eject button, and gently wiggle the blank CD around. If this is unsuccessful, it helps to apply gently pressure to the stereo unit itself or to the dash above the stereo.