This is caused by having the air con drain pipe coming loose. A passeger' foot can cause this. Take a look, it's a 30 second job that a school boy could handle. I'm a ML driver as well as a driver of the greatest Benz model, the 6.3. The 6.3 has the drain line in exactly the same place, this is the only reason I was able to repair the ML on my own. Tell me how the install went.