1997 Plymouth Neon How do I change the mileage sensor

My speedometer kept going up and down and finally doesn't move at all. I went and purchased a new sensor and was told it is by the transmission/ transaxle. My brother tried to find the old oen to change and can't locate it. I have no owners manual so can't find a picture of where it is located. Help me if you can tell me where to locate it so I can get it changed.