The simplest way to turn a car cassette player into a CD player is to buy a converter that installs in the cassette compartment of the existing stereo in your dashboard. The converter allows you to connect to a portable CD player that can be used with your current stereo system and played through the car speakers.
Insert the converter, which is shaped like a cassette tape, into the cassette slot of your car stereo. The converter should be pushed in completely so that it locks into place like a normal cassette.
The converter is attached to a wire terminating in an audio plug on the other side. Insert the plug into the audio out or headphone jack on a portable CD player.
Connect the electric adaptor that comes with your converter kit to the AC power jack on the portable CD player. Plug the other end of the electric adaptor into the car's cigarette lighter socket or 12-volt power supply.
Insert a CD in the portable player and start playback.
Start the car and turn on the cassette stereo on your dashboard, adjusting the volume as desired.
Cassette-to-CD converter kits are available at electronics stores, big-box retailers and auto parts stores.