The rear window defogger or demister was an option on XW, XY and XA Falcons, and consisted of an electric fan that blew across the rear window from the parcel shelf. On the XB, and later Falcons, it was replaced by the heated rear window.
Here is the page from the XA accessories catalogue.
And the section in closeup.
Many thanks to Luke in Australia for sending us these detailed photos of the demister assembly which he has out of a car.
There are several points of interest in the photos. The two inpection stamps at different dates - almost two months apart, the fact that the fan draws in through centre of the motor for heat then into a plenum chamber where it cools the variable speed resistor and then expells through angled vains in the parcle shelf ‘plate’ to cover as much of the rear screen as possible. Plus, the yellow and red factory wire.
It is worth noting that the defogger-demister fitted to XW and XY’s is of a different shape - it is much more ‘square’ with straight side - than that in the XA pictured above. Here is the section from the XY accessories catalogue.
And from Andrews XY GS.
As mentioned above, for the XB the fan was replaced with what we more recognise these days.