The most likely source of water is rain or melting snow, that would normally drain off the car through the drains near the hood hinges. These drains can be plugged with things like rodent nests, acorn shells, leaves, pine needles and so on. I did not see what model MB you have, but they all seem to have a drain under the hinge pocket, and the flow of draining water off the roof, windshield and hood (when the car is moving) can cause the debris on these surfaces to get washed to this area. Open the hood to the vertical positon and inspect for debris, or if the car was sitting for a while, nests from rodents and the like. Clean it out and run water in the area to see if the drain is clogged. If it is you can run a stiff wire, like a hanger, through the rubber grommet to clean it out.
If you have a sunroof there are drains in the A pillars and C pillars that can get clogged from having the roof open under trees and having debris fall in. The A pillar drains can be seen by looking in front and outboard of the sunroof openning, and these can be cleared with either compressed air or some kind of stiff wire. Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)