Well that's a new one!! I have had water in just about every place it shouldn't be on my 123 , but it never got into my trunk lid. I'm not sure on your car but the sunroof rear drains on my 123 are out the C-pillar vents.I wonder if there is a way for it to get into the lid from the edges, the water is channeled around the trunk lip and might make its way through a rust hole or seam ??? You might try pouring water only on the trunk seal area and see if it's coming in that way ? Isn't it fun living on the rainy west coast, we don't tan ,we rust
1985 380SE- 160,000km 'Blistu' - Sold
1988 300CE- Eva,my garage mistress
2007 R350 4matic- The Black Pearl
2011 Ford Ranger-Work truck
1974 25' Trojan Sea Raider- 'Grannyknot'
2002 215 cm Atomic DH skis- Canadian SDH record holder, 175.5km/h- Vars 2010