To run a flash/hardisk Mp3 player in your car you have to have a cassette player. Actually there are two ways.
1) Play a Cd in a MP3 CD player.
2) Play MP3 harddisk player via cassette.
You will have to buy an mp3 CD player for your car which will cost you around 6000~7000 basic. But then there is the problem of changing and buying new CDs.
Harddisk MP3 players contain hundreds or even thousands of songs that you can save in them from your PC. You dont have to spend more time and money to have more songs. Just plug it to your computer and add or remove files. For this you need a cassete adaptor.
and an Mp3 player.