I need to know how to set the garage door opener on the mirror to open and close the door, we are down to only one opener

 

Depends on the opener. If you have one that automatically changes the code you need to find the program button on the opener, press it and at the same time press one of the buttons on the mirror. Otherwise, press the remote opener and the mirror button at the same time until the mirror light goes on. All this is covered in the Operators Manual that came with the car.