Unable to program Prius and Chamberlain Remote together

I have a Toyota Prius 2009 model. I've had this car programmed before to a garage door, but I think the garage opener got replaced with a Chamberlain. I've watched many videos and read the manual, and tried pressing the first remote button on the Chamberlain remote and pressing the rear-view mirror button. It does not program. I tried the "Learn" button on the garage opener too, and it doesn't program or blink the lightbulb like it should when it finishes programming. I pressed the button on the mirror for 2 seconds until it stops blinking and until it starts blinking again with the remote, but does not begin to rapidly flash. I reset the garage door opener on the mirror and it still does not program. My garage door opener is a model HD420EV, and since I've moved back in, I was not able to find a manual for the model. Any help?