Ray,
I would second the helicoil option. In lieu of that, take a tap and tap it out (doesn't really matter what one) to get some threads in there. JB weld into the hole pushing it into the threads. Once it has setup and cured, re-drill and tap to the correct size for the mirror. Paint the top black and you will never know it was done. When tapping JB Weld, make sure you use tap oil and do lots of back threading.
ray