Ford KA - Wet rear footwell

[MariaA]

I have just purchased a lovely little ford KA for my daughter who is learning to drive.  When I bought it, the trader told me they had cleaned the car inside, which is why all the seats were damp, seemed fair enough.  We spent time drying it all out after we bought it, but then the next day all the seats are damp again.  I had put my daughters L-Plates on the carpet behind the drivers seat, and when I picked them up, they were wet, and sure enough the carpet there is wet too.  All the other areas seem dry enough.  I have read various posts about the rear light clusters possibly causing this, but from what I understand, they leak into the boot first, I have checked the boot, under the carpet and under the fake spare wheel section and it all seems dry.  Could it still be the light causing the leak, but it goes straight into the car footwell?  Or is it likely to be something else?  I'm not at all familiar with cars, but would like to try and fix this myself, without the high prices a garage would charge to investigate and repair.  Thanks. 

[unofix]

In these temperatures and high humidity levels it could easy take a week or more for the car to dry out if the seats and carpets have been cleaned with a wetvac. The only way to speed things up and get everything nice and dry is to put a dehumidifier unit in the car for 24 hours or until it stops pulling water out the air.

I live next door to a motor trader and you would be amazed at just how much water they use when cleaning the inside of vehicles. When the weather is warm they always park them with the windows wide open but at this time of year they are using dehumidifiers.