If the there is no signs of rusting in the boot area, it may well be the rubber sealing strip which isn't keeping out the water, either through damage, wear or because the door is not correctly adjusted.
I would start by carefully inspecting the seal, all of the way round. What sometimes happens in freezing weather, is that water freezes and 'welds' the seal to the door, then, when you open the door, the seal is torn. I always sparingly, wipe my door seals over with silicone lubricant before winter sets in.
If the seal seems OK, use a strip of polythene (or even strong paper), attempting to trap it between the seal and the mating surface for the full circumference of the door.
If at any point, you are unable to feel any resistance as you attempt to pull out the plastic/paper, that is probably the point at which the water is getting in.
New seals are quite expensive - it may be that it is possible to make a slight adjustment to the door in order to correct any problems.
What I didn't mention was the possibility of water ingress via the rear lamp fittings or the actual glass seal to the door.
There is even a SLIGHT possibility of it finding a way in via the wiper mounting.
Have a look as well inside the fuel filler flap.