odometer and trip odometer quit working

We've got a '97 Expedition that has a tempermental odo/trip odo and my '91 Marquis just stopped one day for no apparent reason. I took the cluster apart on the Marquis, and the odo drive motors were working, but the driven gears had several teeth missing. As I examined them, several more teeth came off. It appears that the plastic the gears are made of just crumbled from age.

You might check to see if there's a speedo shop in your area, they may have a source for the parts to fix it if that's the case. A law enforcement agency should be able to refer you to a speedo shop in your area.