look for a bad earth, had this on a Vectra once, selected anything electrical and something else would happen, different every time. Drove a trained electrician almost to tears before he found it in the fuse box.
Newbie here, and I have exactly the same problem with my 6 month old Fiat 500L Cross (damn not trying out the rear screen heater when I bought it in the summer !! )
Its been in the repair garage under the diagnostic, no joy.... and its going back in for 3 days in January for a more thorough check.
Anyway, long story short...... did they ever find out what caused your problem? .... ours also shows the right front door open on the dashboard screen, and the interior lights come on, whenever we turn on the rear screen heater.
Weird huh especially since I also live in Wales !!!!
I've got the same problem on a 2014 500L
traced the problem to a broken black wire in the passanger door loom in the rubber boot that goes from the door to the a post