Here's a post about it which I believe helps answer your question.
add-powered-rear-view-mirror-with-compass-t307929.html
I called an installation shop who sells the Homelink mirror. I was quoted $600 which included the mirror, installation and sales tax. It was too expensive, so I passed.
Haven't checked to see how much the dealership charges, but from the above post, others have had the dealership install theirs.
I already have an electric mirror that came with the car, so I know electricity is there. I just didn't think it would cost so much.
