3M makes an adhesive for molding and things like your mirror, it is also used for wheather stripping. It comes in a metal squeeze tube and is yellow in color. It is very stringy and When you apply it wait for it to tack up before you reattach the mirror. Secure it with a few pieces of masking tape until the adhesive cures. Then you should have no problems. It has directions with it..
I did. They suggested Permatex 2 ton epoxy so I will be trying that. Seems like a little over kill, 2 ton epoxy for a 6 ounce mirror, but as long as it holds through all the vibration and bumps while traveling, I will be happy.