Problem solved! Turns out the rubber for the trunk's wiringloom had been pushed slightly in, allowing water to seep in and end up at the exact same location. Also took off both the bump stops, turns out somehow Left and Right got mixed up. Re-installed them properly with some fresh sealant to make sure no water will even get in here again and fixed the wiring loom rubber as well.

@jsoto: I noticed this as well but it looks bigger on the photo. The rubber piece seals this off. The bump stops have now been re-tightened and should be good to go!

Thanks for all the help guys!

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2002 BMW E53 4.6is - Estoril blue
1984 BMW E30 325 - Zinnoberrot
1964 Chevrolet Chevelle 300 - Meadow green