All BA's have a Mobile Phone feature connector behind the fuse box which has 12v Batt and 12v Acc. There is also a feature connector with an Aux input to your stereo. Ford Dealers can sell you the proper adaptor to connect to this AUX connector, or you can have one made or do it yourself.
USB is out of the question as it is a 'serialised' data cable.
My suggestion would be to construct a power lead for your MP3 unit using the ACC and GND wires from the mobile phone feature plug to charge the unit while in use and use the AUX connector for the audio input. A suitable regulator would need to be used on the phone power connector to step down the power to the correct voltage of your MP3 player.
The audio connector can plug into the 3.5mm jack on your MP3 player, which you can tie the GND trigger wire for the AUX connector into (see below links) which, if you charge the unit at the same time, will automatically switch the Head Unit to AUX IN when you plug in the MP3 player.
The information on these connectors can be found on:
(Making your own Auxillary Audio Input for the BA Falcon - Under Vehicle Mods)