Anser,
I had a similar issue with my '07 987CS, not sure if your car has the same electrical box, but drove me nuts until accidentally found the culprit.
It was the No. 4 (positive cable from the battery to the distribution box), where it connects to the distribution box at the firewall, it's fairly easy to check.
PS: No. 11 is inside the cabin, behind the glove box, but the connector side is on the front trunk..
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