When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Basically, you stick the mirror (facedown) in a pan of gasoline or mineral spirits and leave it overnight. That dislodges the glue. You need to make a couple of thin [non-rusting] sheet metal shims to stick under the swedged retaining heads to tighten stuff up. Then put mirror face back on with RTV (do this with face down...if you do it the other way, the mirror will warp a bit due to its own weight and the reflection will be distorted).
If this is for the right-side mirror, you might want to consider getting a newer (and a bit larger) mirror from a junk yard, then cutting it down to the correct size. That will give you a 'bubble' mirror which is MUCH better to see with.
I read the tip on here recently about using a $4 pressure pin punch on the ball of the base. It took me all of 30 seconds and worked GREAT! I just used it on the ball at 3 or 4 different spots, under where the mirror would swivel on top of it. It's been wind tested up to 75 mph so far and it holds tight.