The only way that I can think of is to buy one of those fm transmitters. Connect your mp3 player to it then tune your stereo to the fm transmitters frequency. A lot of sound quality gets lost along the way though.
It would be a lot easier to buy a stereo with an aux in plug or better still a usb plug. You can pick a good second hand one up for less than 50 quid and the fascia adaptor and fitting kit for less than 15 quid. The sound quality would then be vastly improved over the standard Ford rubbish.