I've fitted power points under the rear seats to power heated seat covers, powered from an ignition live in the rear fuse box.
Also fitetd a 30amp supply for a plug in heater also under the rear seats, also powered from the rear fuse box. Infact I've just put the heater in as its dropping cold. Fitted the heater when I got the car as Ford in their cost cutting had removed the supplementary electric ceramic heater in the diesel build from 2016 (or possibly late 2015) onward.
It was an electric supplementary heater that was incorporated within the cab heater. On the Mk1 Kuag it was approximately a 600W heater, in the 2012 onward Mk2 it was a possibly a 300W heater and from 2016, no heater. Made all the different to defrosting all windows. Replaced with a heated steering wheel, which must help a great deal in defrosting side windows.