ok put your key in the ignition. hold your button on the dash that controls your odometer...you know you push it and the LED changes from trip meter to odometer...hold that down then turn your car to on or crank it. hold this down until you see test come up and let go.
when you let go it says gauge and the needles sweep all the way to the max then back to 0. then press again all bulbs light up, press again and get the ROM version. Then EE level, hex value for mfg date, then the next time you will get the diagnostic code. (press 6 times i think). its going to say dtc then a 4 digit code.
whats cool is next. you press one more time you see ENG0000 and this tells you your mph! Then metric, tach, and a few other things. even tells you your exact fuel level, oil pressure, few other things.
for a list of all the codes and pics
http://www.mustangworld.com/ourpics/fcar/dtcodes.htm