I know you can turn it off...and I am usually very aware of everything going on with my car, but I am amazed at how often I really do forget! I will look at realize that this thing has been on for 1/2 hour or more! Cant be good...

I know the feeling, but I also don't think they get hot enough to damage themselves.

Of course, someone will be along shortly with a story to prove me wrong...

I would guess they're designed for continous duty with having that feature of remaining "on" indefinitely at speeds greater than 30mph. But like everything else, they have a service life.

The reason for the timer 'feature' is to not let the battery run down. The rear defrost draws 30A so they didnt want it on too long while idling. If your up over 30 mph then your building enough charge for the battery and the defrost. The gris is just a resistor bank so it cant really wear out but as Mos pointed out it is suspect to mechanical and uv damage.

Yay for me, I was at least partly right!!