Don't need to take any of the mirror housing apart. The glass should be attached onto the heated backing pad which just unclips. Pull the glass off, unclip the backing pad, remove the heater wires and take your 2 bits indoors to glue back together. I used Loctite Serious Glue to stick mine back on and it has been fine since.
They fall off because VW thought it'd be a good idea to have a rotating switch for the heated mirrors, rather than a button with a timer. You turn the switch on and then forget about it until 3 months later when the sticky pad has cooked and your mirror starts doing the wobbly!
