2003 Chevy Blazer Do all zr2 Blazer flood easy?

I have a 2003 Chevy Blazer zr2.
I have just purchased it. The car is in very good condition. However, when it rains it leaks alot. The sunroof is hard to tell when it is closed all the way.
However, my main question for you is that last night I had very hard rain fall, and in the back of my truck when I turn I hear water swooshing around in the back. The mats in my car are all dry, and it wasn't the sunroof this time. What could it possibly be?