Rear View Mirror Removal

Can someone tell me how to take the passenger side rearview mirror apart. The truck was apparently vandalized as I cannot see how this type of damage
could be done any other way except by pushing up on the underside of the mirror housing. The mirrow just swings forwards and backwards freely, it looks like the device or part that holds it firmly in the bracket is broken.
The truck is a 2000 Chevy LS extended cab.
Thanks for any suggestions.

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