I've found the culprit and thought I'd share as I've never hear this one before. The water leaking into my truck was as a result of the sunroof drain hose...no, was not clogged. The water was coming in the passenger front foot well. The front passenger sunroof drain hose comes from the sun roof down the A pillar and then snakes out the passenger compartment through a grommeted hole in the body side below the A pillar. Typically, water from rain drains down the sides of the lower right and left of the windshield (not only through the drain holes in the rain tray) and down the outside of the A pillars. Some of this water also drains into the inside of the fenders inside and behind the wheel well housing shielding. This water drips onto the area of the grommet from the sunroof drain. This water was seeping around this worn grommet going into the passenger compartment.

Mercedes had designed the external portion of the sunroof drain with a long 6" piece of tubing, this flaps up and down and has resulted in the grommet moving up and down slightly and therefore no longer creating a proper seal.

Remove the wheel well shielding and silicone around this grommet and it will solve the problem. Also you can shorten this pathetically long drain hose to a more reasonable 1-2" so it won't bounce and cause this wear. I did both sides - so far all it dry.

That will fix it! Lot's of money saved and a mystery solved with the cost of a tube of silicone. Would not want to know what the Mercedes stealership would want to replace the sunroof drain hose - is a bigger job than you think.