I would just replace it. I dont think your going to have a fix for this, besides a new one. Its just old and warn out. If the car has to be moved, you can always take a good straight blade screw driver and just pop off the thumb piece, then pry out the key part, and turn it over yourself, like you would if you where going to steal it. I am not sure what a new one will cost, or where to get it.