Heater unit in engine bay removed. This is the rust that was underneath it, so the heater unit stood on top of this. Rain water entered the ventilation flap in front of the windscreen, and trickled through the heater unit, leaving via a hole in the bottom of the unit. Visible on the photo is a recess where the water was collected. It then leaves via the hole (lower right, behind the '2011/' text) and drips down to the road.