I have an 2001 vectra with similar problem but in the rear foot well?

I assume water leaking in to the cab floor. Could be the door seal. However try putting a dye in the screenwash bottle something that does not stain or
just use the blue screen wash fluid and set up a bowl where the leak is to
catch and see if its blue then it could be the pipe has separated from the pipe joiner.

Bob
February 2014