how do you turn the overdrive on and off

If your light is not going on and off when you press the button, one of three things is happening: 1) the button itself stopped working. 2) The light that illuminates is burned out. 3) the transmission has a problem. You can eliminate #2 if you are crusing at 50mph, you press the button and you feel the transmission rev up a little bit. This means the O/D off selection is working, and the light is just out. Other than that, you really need to hook up diagnostic equipment to monitor that button and the transmission response. Good luck.