I don't want to add confusion here but I note that you have an S2 coupé ... like me. I also noticed your earlier picture querying your switch was of the area by the air vent not by the heater controls.
Now on my S2 that switch isn't the rear demister ... it's to manually switch on the engine fan. My rear demist appears
above that by the heater controls. Below is a picture to illustrate what I mean!
Anyway, if the button is doing diddly-squat at the moment then, as David says, you can use it as you wish. There must be some 'James Bond' type gadget that would suit a Fulvia!
