If you can find a wireless FM modulator for your MP3, that is not a bad way to go. We have a wireless FM modulator for Sirius radio, and the quality is decent--not digital, of course, but as good as an FM radio station. It sounds different in each vehicle, but the quality is tolerable (especially when the alternatives on local radio are totally unacceptable).

Alternately, why don't you burn CDs from your MP3 collection? Should they be damaged from the elements (doubtful), you can just redo them.