Holy cow. I have a 97 Mustang and had the *same* issue 2 years ago. Ended up having to replace the instrument cluster as well. They ended up getting a part from a junkyard for me. Seems the gears that drive the odometer, spedometer, etc are made of plastic and the teeth break off. Love my Mustang tho :)

Now I just need to get the power seat fixed... Seems it no longer wants to move forward, which is a problem when you drive a manual transmission. I'm short and if any one moves the seat I can't reach the clutch!