Buy a Cassette Adapter
Believe it or not, CD players were slow to replace cassette players. Cassettes did have a nice run, and a lot of people simply wanted to continue listening to their cassette tapes. After all, a lot of people spent a significant amount of money building a cassette tape collection they could be proud of. When CD players like the Sony Discman D-50 first hit the scene, manufacturers quickly designed adapters for cassette players that would enable you to play your CDs (on a portable CD player, of course) through the cassette slot. This method still works today and kits can be found online for reasonable prices.
Some kits come complete with a mounting bracket that allows you to fasten your portable CD player securely within your vehicle. The adapters look just like a cassette tape with a plug that fits into the headphone jack of your portable device. You simply insert the tape adapter into your in-car cassette deck and that's it: Your CD music plays over your vehicle's speakers as if it were a built-in part of your car's audio system.
Most kits also come with a small AC adapter. If your portable CD player has an AC input, you can plug one end of the adapter into your portable player and the other end into your vehicle's cigarette lighter and you have a power supply. No more wasting batteries.