Hi

Pretty sure it comes in via capillary action via washer pump.
When I did my first one of these , i doubled checked for leaks as im sure you have done, it was back in six months, same fault etc.

Found by luck rather than judgement as customer had been using a bright pink cherry scented screenwash from tesco and had poured it in neat, the fuse board stunk of it!!

I believe the washer pump has be superseded from Ford also!!

Everyone(and the original as above) ive fitted washer pump since, I believe the washer bottle level can also cause it in the higher spec cars

MARK