Had the same problem on my 2002 Mondeo.
It was caused by the rear screen washer.
If you remove the interior c-pillar above the rear seat
you should notice that the washer hose has an inline connection, that connection worked loose on mine so just pushed it securely together again. I also noticed a stain on the interior c-pillar. Also lift your rear seat up and see if it's damp there.
Give that a go!
Pete cheshire, December 2008