[sick472]

A few options are Craft Spray Adhesive (3M is the best I have used), thin double sided foam tape (3M for sure here), and down and dirty...some caulk silicone. Silicone holds replacement side mirrors on very well, but the double sided tape has the highest vote in my opinion.

And, make sure you get the old glue off if using the spray adhesive or tape. The silicone would prefer to be a nice thin layer too, but would be more forgiving if not. Either way the install will look much better without the old goo keeping if from being nice and flush and flat.