Longbox is right on, I did this, BUT, I suggest only drilling one 1/8" hole from back, then place mirror base on and mark the other two and then drill them with 1/8 from front, then final holes from front also. The "guide" clearance holes are large and you would have to find the exact middle in such a hard to reach place. You will get a better fit for those unforgiving csk head type screws. So just drill the easiest one from back.
