More than likely it will be the small black nylon gears that have eroded from age, the trip odometer will stop too, if not already from the same thing. You should have found some very small pieces of black on your floorboard, it looks like dirt so it is often overlooked. You take the dash apart remove the cluster assembly, disconnecting it and you will likely see lots of gear teeth. I bought mine from a guy that makes them locally. Mine have been working ever since replacing. Website is : www.odometergears.com . Just be careful replacing them and not to strong handed.