You need both switched and unswitched power for the mirror technically, although switched is actually enough. The reason is that the Homelink works with the ignition off, but the light, auto-dim (which it has) and compass you do not want on when the ignition is off. It wants both power leads.
If I remember correctly I got an adapter for the mirror mount as I didn't want to screw with trying to take the other one off, and that was successful. There is only ONE manufacturer for Homelink mirrors -- they have a patent, so no matter where you get it they're all made by one company (which is why they all look mostly the same too.)
OE VW has the Homelink buttons and transmitter on the sunvisor. I couldn't find one at the local scrapyards, so I went that direction with the mirror some 10+ years ago, and like it a lot. The auto-dimming rear view mirror is a nice bonus doing it this way.