I may be confused....but do you mean you want to connect an MP3 player to your car radio via FM ?

If so, there are plenty of third party adapters for iPod. Most use the iPod proprietary connector for power but ones which only use the player's headphone socket will work with other MP3 players.

I'm not sure I follow your point about wireless stations, though I could see there might be a problem with very powerful stations interfering as you move from one locality to another.