I have one of those little "i Rock" FM transmitters that'll plug into anything with a headset jack and run on 2 AAA batteries.
Between the XM radio, regular Blaupunkt Florida CD player, the 20 gig Ipod using the 'irock' and the Creative Muvo mp3/wma player (doubles as USB drive) also plugged in to the 'irock', I'm completely covered.

It also helps that I listen to a lot of Old Time Radio in mp3 with low bitrates, so audiophile quality over the clattering Diesel is not a priority.

I've found that the WMA works better in that I can put more on a device using 'standard' settings. Mp3/WMA...I don't hear much of a difference at all. It's not like I'm listening with high quality headsets.
I'm someone who grew up grateful to have a car with any kind of working music source. A 6'" across oval speaker in the center of the dash playing FM in mono was a luxury.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."