can you find a speed and mileage cluster?

There are a number of things that could be wrong. It could be something as simple as a bad connection in the dash that simply needs to be glued or soldered back on or just dirt or grit stuck in the gear that is preventing it from turning (I'm assuming you have the old "click type" odometer where the numbers are turned by a gear.) Or it could be an electrical failure that is preventing your odometer from receiving a signal but that usually means that the speedometer won't work either. If the speedometer is working but the odometer isn't it probably means that the connection to the odometer is loose. It requires removing the dashborad, and sometimes the steerting wheel, to get to the wires in the back. Make sure the connections are good and possibly use apoxy or glue to secure the connection.

I have seen this happen before and if one works and the other doesn't it is not repairable. It has to be replaced. And I have one out of a convertible if you are interested but any mustang 92-96 will work.