Go to a home improvement store and get some some Gorilla "Construction" glue. If that doesn't hold it nothing will. What I like do for things like this is mix that with like 1/4 part of super glue but don't actually mix it just put a light coat of super glue with like a toothpick or something on both sides and then the construction stuff is really thick like a paste and a good glob of that on there just don't put too much or it'll look funny and might not hold up as well. Tricky part is you going to have to hold there while the glue cures and your not going to be able to drive like that for at least a day and a half... and that's the part your gonna have to figure out because this isn't glue&forget. Then it should g2g for a long while. I fixed a porcelain mug handle like this and it eventually came apart but I put absolute screws to it freezing the mug and unfreezing the mug over & over & over & over for like that a year or two before it finally came off. Lastly it's not gonna hold forever because your worst enemy isn't the temp its road vibration so it'll jiggle off but that should hold in place for a long time. GL figuring out how to brace that thing while the glue cures xD that probably why it's come off so easy before. Give it atleast 36 hours to cure just to be safe since your mixing glues. GL with the mirror dude   

Edit: forgot a thing. where ever the break is on both sides scratch up the surface really good before you glue. That'll give it the best to stay up.  

What kind of glue are you using?  If you haven't tried already, maybe try a healthy dose of Gorilla glue.  Also make sure the area you're gluing to is clean and sufficient to hold the weight of the mirror (so I mean don't try to glue at a hinge or pivot point--that won't really hold up).  You could also try to just get an aftermarket mirror and glue it below or next to where the original mirror was sitting.