In my 1996 Excel Sprint, when it rains, even when the car is not moving, a pool of water builds up at the left hand side, on the floor of the back seat. I can't find any leaks, and the water isn't trickling down from anywhere, not even the gap in the door. It just builds and builds until it's about 1 or 2 cm high, and takes days to either evaporate or escape. Is there any place near there which would be a possible place where water could be getting in?

we have the same prob with our mercedes from 1973, funny enough it's also the left handed side, but lucky for us not that dramaticly.
If we find what is wrong I'll let you know ;)