In my experience, leaks of this nature (onto the floors) are almost always from the windshield or rear window weatherstrips. Sometimes the leak is between the glass and the seal and moisture can be seen tracking across the seal at the bottom, particularly at the corners. Other times, the leak is between the body flange and the seal and can't be seen as easily. I believe 3M and Wurth make a sealant that can be gunned into the areas described. If that doesn't work a new seal is called for. I've seen this on almost all M/B chassis with rubber window seals.