I could explain it in one sentence. But for those who are still in need of help here is my short story. And, of course, you CANNOT use headphones in a car. It's forbidden and dangerous.
I own a relatively old iPod mini, one of those first color cool looking iPods. They are not for sale anymore, some sort of rarity. But for the purpose of this discussion it doesn't matter the type of of your mp3 player. It has to have a headphone jack output which all of them have.
Now, there are many different devices that allow you to connect between your mp3 player and a car stereo. Basically they fall into two categories: wired and wireless. Some new cars have built-in interface for iPod, but I'm not familiar with that. My car is old.
So, I've tried two wireless devices from Belkin and RCA. You need to connect a transmitter to the iPod, or what you have. Then you find on the car radio on specified frequency your music or whatever you're listening to. By the way, I'm not listening to the music. I prefer spoken word, such as podcasts or audio books. For example, recently I was enjoying listening to Stephen King's Green Mile. You can download it from www.audiobookzilla.com, they have a huge selection of audio books.
But, back to our transmitter. I did not like the quality of the sound in both of them. Maybe it's just my car, I don't know. Instead, I've tried a cheap Phillips cassette adapter kit. You just need to play the cassette, and the sound is "magically" appearing in your stereo. Quality is much better, then in radio adapters. And, also important, it is much cheaper. The only thing that you have to have a cassette player in the car.