Automatically turn on the rear defroster when needed. This might be connected to the rain sensor (i.e. high humidity) and temperature to gauge when the rear window will fog up.
There is no wiper on the rear window, as a result of which rear vision is obscured when driving slowly in rain. Turning on the defroster can mitigate this problem, but the defroster is slow to clear the window. This is only temporary and on a long drive the driver needs to monitor the defroster and turn it on again when its timer turns it off automatically to avoid having to deal with a build up of water or condensation.
This is especially an issue in city driving, where all round vision is needed continuously. It is less of an issue when driving at sustained highway speeds as rain on the outside has less chance to collect on the rear window.
(lightly edited by moderator)