My car has the same problem in extremely cold weather, once it warms up, it works again like it should. I would suggest cleaning the switch, but I don't know if that will help. It is possible that the metal connectors that are engaged bend away from the contact point in the low temperature in my case that is. In your case, if it happens all the time, it might be a problem with the relay. Come to think of it, the relay is what holds the crrent flowing through the circuit since the defroster is automatically switched off after a period of time, so it would probably be better to check the relay instead. I'd check both
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