This normally happens when the rear wiper is is blocked and people keep pressing the rear screen clean,water then forces it way out of the motor and on to the tailgate micro switch which is in the catch from memory,if you take off the interior rear trim it might have water all over the inside of the tailgate as this happens a lot in winter when the rear wiper blocks/freezes up and happens on most of the vag range.
If it doesn't go inside the tailgate the washer pipe can also blow itself apart on the connector inside the rubber boot between the tailgate and roof panel and when this happens water normally wets through the car such as roof lining and drivers side 'C' pillar trim,then down at the side of the rear seat and seat base.