I've tracked down the reason for my windows misting up internally - the carpet behind the drivers seat is sodden. Clearly water is getting in somewhere.  Any suggestions?

Morning Andy

Welcome to the Forum

First check the smell of the water as I have found the rear washer tube split above the B pillar then washer fluid ran down the B pillar every time the washer was used, strange the washer still worked just not as well as it should. If its that then you should smell washer fluid.

Check sun roof drains they are regular culprits for leaks, also check the rear door drains and the membrane behind the door card water often runs down the glass and if the drain is blocked and it can get past the membrane by splits or it not being sealed properly it is scary just how much can come in. 

Good Luck with the hunt hope this helps

Dave