you need speakers that can fit into thelittle socket in your MP3 player, unless tehres a converter of somekind.
you might be able to get a "CD" sort of thing, like you can with a cassette and there is a wire coming out and that plugs in to the headphones plug, don't know if you can get it for cd players though !!thecraigc!!
get an FM transmiter on radio shack for about 30$ u just connect the player to it and set the frequency on ur car radio to the same frequency on ur fm transmiter