Have a chat with your friendly, local windscreen man Mike.. They'll know the best stuff to use..
Once upon a time you could get the special glue needed from factors but I can't recall seeing it recently... Even if it is still available I'll expect the pukka stuff used by pro windscreen fitters will be many times more effective and better.
Windscreen glue should be used, but (a) it's expensive -especially for a one-time-only job- and (b) needs extra care when applying, so an experienced windscreen shop should be a better option
Just one thing - as you don't get a lot of glue - make sure you really clean the interior glass and the back of the mirror metal base first to get any grease off.
Please note, I'm no longer active on the Forum, so won't respond to messages.
Agree with the cleaning, I read a real proper academic study somewhere that concluded that most adhesive failures are the result of poor cleaning, good glue is pointless if the surfaces aren't prepared.
Lots of adhesives appear to stick very quickly but full strength chemical bonding takes longer. Ideally do the job when you won't be driving around for a few hours and adding load to the mirror as you leap from pothole to pothole. I remember doing one ages ago on a car, used the proper stuff and cleaned with alcohol, left it overnight and I was surprised to find that it stayed stuck "forever".
Richard_C
Current:C4 Max Hybrid, C3 1.2 Auto
Past Citroens: Dyane (x2), 2CV, Visa, BX (x2), Xantia, Xsara Picasso, C4 Picasso,C3 (x2) C5 X7 Tourer, Synergie 1.9TD, C1
Others: Hillman Hunter, Cortina Mk1, Maxi, VWT2, Granada, SAAB 900, SAAB 9-5, R5 Gordini