[smileynialley]

You have to take the marker lights off and look into the bulb holder for dampness...mine was so subtly letting water seep in you wouldn't notice with a test but after heavy rainfall the interior would be drenched.
the water would come in through the roof lights and travel down the windscreen pillar to the floor menaing the carpet would be damp.
try taking the windscreen pillar surround off (unclip the handle and the whole thing will come off) and feel the metal work for water marks.
If not, try taking up the carpet and looking for any holes etc in the body work.