I'm not 100% certain of your situation, but I do know that there is a glue, specifically called "rearview mirror" adhesive. It's made for that one and only purpose. I've see it at Walmart, but I'm certain all the standard autoparts places sell it as well. Check it out and follow the instructions.
Here's one example from Permatex: http://www.accessconnect.com/r...e.htm I have not used their rearview adhesive but I've used their windshield repair and it was amazing!!!
Here's popular mechanics version of "how to do it": http://www.popularmechanics.co....html
Hopefully that will get you going