Leaky ES

I've had my '02 Galant ES for about a year now. Luckily, it's been a pretty dry year (not luckily for the farmers, of course not). But everytime it rains, the floor gets soaked. At first, it was only the backseat passenger side that got wet. Now recently, it is the entire passenger side floor. I cannot find the source of the leak, try though as I may. Like I say, it only happens when it rains. Anybody else have this problem?

I must add that other than this anomaly I have been very satisfied with this car.:)