Mark

Three things normally end a window's life. Both temperature sensors go open, the heater element goes open, or the window cracks.

None of these are uncommon.

It looks like yours cracked. I wouldn't blame the controller as I have never seen one to apply voltage beyond what is required to keep the window warm.

On a side note: Why are window's heated? Beyond the obvious of keeping visibility clear...... window heat keeps it pliable. When a bird hits it, the window will flex and hopefully let the bird bounce off. If the window was cold or frozen, the window will shatter and possibly have the bird come through.