Only one line in rear demister works

[Dracoro]

Hi

Middle line in rear screen works when demisting (frozen window this morning, so a good test!).
All the others do nothing.

Any ideas on cause? A cursory glance didn't reveal any obvious broken lines that I could see.

Car is Porsche 987 Cayman if that’s relevant…

[GreenV8S]

It'll be a break in the other tracks, or the part of the heater element leading to or from them. There really isn't anything else it could be, given that one of the tracks is working.