T350c rear de-mister issue

[Talksteer]

My T350c rear screen de-mister doesn't work properly.

When pressed the light comes on and according to the garage current flows through the cables. However the light only stays on for 15 seconds or so, when it is meant to stay on for about 3 minutes.

Has anybody else had this problem, and if so how did you fix it?

[Targarama]

Mine doesn't stay on for long, I'm sure its more than a few seconds though. I think TADTS to some extent.