This happened to me before aswell. But in my case when i pressed the defrost button, the light wouldn't light on either, let alone work. It turned out that it was the relay, this relay also caughed turning signal problems sometimes aswell. So if you have turning signal problems aswell, check the relay...

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My Cars
Mercedes CLS
Mercedes C-Class W203 - All time favorite

R.I.P.
Mercedes C-Class - W202 Lemon
Mercedes ML - Warrior
VW Jetta – Worst Car I ever owned - Alot of problems