The year of your car is important, there are two versions. The second version has a small resistor on the back, they are not interchangeable, they are stamped 1969 and 1972 I think. Remove it and gently pry open the case using small screwdrivers. You need to bend the arm that travels in a track so that it catches when pushed in, this is a very common failure. Or just stick a matchstick next to the button.