The main function and purpose for roof side weather strip is to protect wind noise in driving. The water penetration is taken care of by Door opening weather strip. It is guess that the reason for water penetration might be due to door poor assembling, failure in door sash area, failure or malfunction in door opening weather strip, or poor assembling on door assembly. MFTBC recommends to investigate door opening weather strip (localized parts) or to check assembled condition on the subjected failure area.
Best Regards,