Make it Rock!
The irock!™ Beamit Wireless Music Adapter from irock! is one answer. For around $30, you can broadcast the sound from your portable device into your car radio. Sounds complicated? Nah. This is a piece of cake ...

The Beamit™ itself comes with a 1/8" mini-phone jack. Simply plug it in to the headphone jack of whatever you want to listen to. Tune in to the same FM station that the Beamit™ is set for, and rock on ...
How it works
The Beamit™ is a small FM transmitter. It has a switch on the side which allows you to broadcast on 88.1, 88.3, 88.5, or 88.7 FM. It supports full stereo, so you'll be able to enjoy (or suffer through) the full quality of music coming from you portable device.
Beamit™ is powered by 2 "AAA" batteries (which are not included), or you can use the DC car adapter. Use of the car adapter does not require batteries to be installed.
Does it work?
Yes. We have a TV/VCR combo in the van, but it had dropped (that's a subject for another review), and the audio is gone. We had limped by using small speakers, such as you would see next to your compter monitor, but the sound was small, and the kids fought over who got to hold them. 
We bought the Beamit™, and had it up and going in 5 minutes.
Does it work well?
We tried all four channel settings, and had no trouble getting reception. There was a high-pitched whining, similar to what you would hear from an AM station during a storm, but we discovered that the kids had "cranked" the sound on the tv ... the only way to hear it with the small speakers we had previously used.
Now with the Beamit™, the volume could be set at a reasonable level, and everyone in the car could hear. Stereo quality was nice.
In fact, the "other" family car (read: Dad's commute vehicle) now has the Beamit™, since it has no CD player. I use my PDA as an MP3 player, and get CD quality sound. Very nice!
Get the Irock!™ Beamit at RadioShack.
irock! website.
And check out the forums too!
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