Nick,I seem to get a lot of condensation in my Venga CRDi since I got it Dec 1, and noticeably more so than in my previous Toyota Avensis. I wonder if is down to the weather or Kia. It seems to build up as I drive since I keep the car in a garage when not in use.
Most common cause is wet carpets and I mean wet ones not just damp. Sometimes from leaking door seals or leaves blocking drain holes around base of windscreen. Or, in hatchbacks and estates a loose connection to the rear screen washers- has been mentioned recentlyin another thread.