Do you have a sunroof?
In a really nasty way ford designed the sunroof rails to drain away into the sills. This is why '80s fords have such bad rust problems with inner and outer sills. Could be that it is running in to the car through a hole in the inner sill.
There is a common mod done to mk2 fiestas where a hole is drilled in the outer sill and the sunroof drain pipes are then extended to poke through the hole.
Lee
