Just read the manual today about this and yes it is supposed to turn off after a specific amount of time. I was researching it because the other day I pressed the button to turn it on and the button got stuck and would not activate the defroster. Once a I got home I sprayed a little WD-40 around the button and it is now unstuck, but now it comes on everytime I start the car and can't be manually turned off.

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2001 X5 3.0i (PP/WP/AP) Traded at 213K miles
RARE White on Black - Why does everyone get Tan??
First E53 with 215s on XOutpost!!