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I had the same issue yesterday on my 08 Cayenne S. The issue was not a bad bulb but a ballast issue. The ballast allowed water to get in. We cleaned and patched up the ballast and siliconed it so its now water tight. Hope this helps others.
I just picked up an 08 CS and have the same issue. Would love to find out if this part can be reattached or if I would have to buy a new key..