Driving in the rain today I noticed that the second heating element down in the rear window of our '10 500c wasn't keeping the glass clear of spray.

It's pretty obvious why: the rear headrests' metal supports have rubbed on the glass and broken the element.
The headrests are up above their highest slot, so we can't really blame Fiat for this - it's just unlucky that the metal support touches the glass right where there's a heating element. We've had the headrests up that high because otherwise the kids' seats don't fit.
So my tip for fellow 500c owners is to remove the headrests entirely if you need to fit child seats in the back. (I don't know whether hatchback 500s would suffer the same problem.) It's slightly inconvenient having to carry loose headrests around but it's preferable to this alternative.
Anybody had any luck repairing these demisting elements in the rear glass?

It's pretty obvious why: the rear headrests' metal supports have rubbed on the glass and broken the element.

The headrests are up above their highest slot, so we can't really blame Fiat for this - it's just unlucky that the metal support touches the glass right where there's a heating element. We've had the headrests up that high because otherwise the kids' seats don't fit.

So my tip for fellow 500c owners is to remove the headrests entirely if you need to fit child seats in the back. (I don't know whether hatchback 500s would suffer the same problem.) It's slightly inconvenient having to carry loose headrests around but it's preferable to this alternative.
Anybody had any luck repairing these demisting elements in the rear glass?