The 2017 E300 is the first car in a few years with a different Homelink system. Here is what worked for me.
Step One: Hold one of the buttons on the bottom of the Mirror (1,2, or 3). Light flashes.
Step Two: At the same time, hold The Button You Want to Program on the remote. Light flashes quickly.
Now your mirror is programmed but you must teach the garage about it. (Love this guy!)
Step Three: Press then release the Learn Button on the Head Unit of your Garage Door Opener. He’s pointing at it.
Step Four: Press and then release the button on the mirror that you programmed (1, 2, or 3). It might work! If not, press and release again. My garage works on the third try.
You did it! Nice work!
Note: There is a know issue with the 2017 E300 not holding the code. If this happens to you, take it to the service department and ask them to “Add the Operational Codes.” The notice was posted to dealers on June 30th, 2016 and takes less than 30 minutes to update in the car.
(Thanks to Chris in SC for helping me solve this mystery!)