If your car audio system accepts an incoming Bluetooth connection (as many/most do these days), you might be able to use a portable CD player that has a Bluetooth output, such as the one below.
However you wouldn't be able to easily skip over a track that you don't like, as you can when a car has a proper CD player built in, because you'd need to take your eyes off the road, and your hands off the wheel, to be able to fiddle about with the player:
Amazon.co.uk User Recommendation