I have done this through the mirror. I used the basic bits in quoted post below to do it, and threaded the wires directly into the back of the mirror plug as shown, only without the metal pins (I may use them later, the ones I had were too short and were not making contact). V1 turns on/off as you'd expect, and it works great. I plan to look at adding in the remote display soon too, which should just be a matter of running the other two lines in the phone line to wherever I decide to mount it.
If you do do this, I highly recommend at least one ziptie keeping your phone cord in place inside the mirror, I had mine slip out the back of the plug once which was annoying.