http://www.mp3-box.org/car-mp3-fm-transmitters/

Can you attach a FM transmitter to your laptop and play MP3 on your (car) stereo?
Can I attach FM transmitter (typically used on MP3 players) to my laptop so I can play MP3s on my car stereo? This would be much cheaper (and require less synching up) than getting a portable mp3 player.
Please advise. Thanks!
I agree with the previous poster, theoretically, yes.
However, in my experience, FM transmitters don’t work very well in crowded metropolitan areas such as the San Francisco Bay Area. You drive a very short way and then need to retune because you get interference from another station.
If your car has a cassette player in it, use a cassette adapter instead.
And, I also agree, laptops are unwieldy, have short battery lives, and are targets for theft. Slam on the brakes and your laptop can go flying.
A portable music device and a cassette adapter would be a better option. It’s what I use. But don’t leave the portable music player out … my wife already had her’s stolen.
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