Many years ago I bought a small battery charged radio transceiver that has a 3.5" male audio adapter. It just plugs into whatever device you have just like headphones would. I used it to listen to books I already had on tape, over a cheap tape player, after I got a car that only had a CD player.
I now use it with my droid and can listen to any media on the droid over the radio speakers. I use it mostly to listen to my downloaded podcasts, but sometimes pandora as well.
It uses a lower bandwidth FM channel and with devices of this type you typically have four to choose from (so you can try to choose one not being used locally to avoid interference).
I bought it from Radio Shack for $25 eight or nine years ago, it runs on AA batteries, for which I just bought rechargeables--which still work great. There are many versions of these now available because of the iPods, but the prices are insane now, presumably because of the increased demand.
If you purchase one, be sure to check the size of the adapter. For the Droid it must 3.5mm (like a tape player) and not 2.5mm as many other phones now use.
It's a brilliant little device, I absolutely love it.
Best,
Hikinthru