You will likely need to pull the dash apart, and open up the cluster to see what is going on. Either the odometer motor is not getting power, or it is not turning.
My bet: Dry heat is brutal on plastic parts. I would almost be willing to bet money that the gears are cracked in the odometer. I have the same issue, luckily mine is on the trip meter. For it to be fixed properly, everything must be properly documented, otherwise flags get thrown for odometer fraud when you try and pass it on to the next buyer.