I will ask my buddy who use to be a glass installer.  He might know the answer.

I would clean everything with something that breaks down silicone.  If silicone gets on anything, even if you can't see it it will prevent bonding.  Typical cleaners don't touch it.  You need something like TSP. 

I had a heck of a time getting the rear view mirror to stay in the BMW we had. That thing weighed a lot more than what’s in the truck. Had my best luck surfacing the back side of the button with fine file and cleaning everything with lacquer thinner.

You shouldn’t have to, but etching the glass to give the adhesive some tooth to bit into might be part of the solution.

I recently had to glue the mirror on my 69.  Am i hearing you correctly that yours is plastic?  If so, I bet nothing will work.  At the very least, sand it flat with some fine sandpaper, but I have never seen anything that sticks to plastic very well in a situation like this.

On mine, which has an aluminum mounting lug, I used the stuff OReilly sells.  They have one kit for like $15 and another that is same brand but only $5.  It doesn't come with some stuff the more expensive version comes with.  I chose the $5 version.  I used the cleaning solution/wipe cloth that came with the kit, then applied their activator to the windshield and coated the lug 100% with their glue.  When I touched them together, it never moved again after that.  I had sanded the aluminum lug so it was very flat with 180 sandpaper.  This was two weeks ago...  I let it sit overnight before mounting the mirror.

If I walk out there today and my mirror is sitting on the console, it is all your fault, sal.

As MS stated, I also let the metal piece sit for a day before mounting the mirror.