Windows will steam up if any water is in the car, even if you soak your carpets with water from your shoes. It's nothing to worry about, as long as you aren't leaking but the footwells will be soaking so that'll be easy to tell.
dkw, I can tell you exactly what it is, like Neil says, your inner door seal.
You need to removed your door card, there are a succession of screws that hold it on along the bottom of it and also two in the door handle (big black chunky rail, not the chrome release handle). For these you need to get a small flat-head screwdriver and remove the front half of the handle by forcing it gently apart.
Once the door card is removed and all the wiring is disconnected, you'll see a large silver plate with foam sealant right around the whole edge. You'll spot the
problem, marked with a dirty water stain where the rain water has been dripping through the seal. Simple losen (but dont remove) the metal plate by losening the bolts that hold it on, then get your silicone sealant gun and reseal the part where the foam sealant has perished. Mine perished at the bottom of the metal plate, so I resealed the whole bottom of it so I don't have the issue again. Since doing this and the 3day of heavy rain we have had this, it's fixed and no longer leaks
!