I had a car that used to do this same thing (although it was not a Benz). Did you happen to notice if it was excessively humid on this paticular day? Condensation will sometimes form on the inside or outside of your hvac ducts and this is usually the source of moisture. The only other possibility I know of is a leaky heater core. The easiest way I know to check for this is wait till the next time you notice the moisture and get some on your fingertip. Now rub your fingers together. If it feels "oily", it could be coolant and that would be your heater core. If it feels like water, it's condensation, and there's nothing to worry about. If you are still unsure try the smell test...

Happy motoring. Paul

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'99 SLK230 (Bahama Blue)
17" O.Z. Racing "Vela" Modular wheels
Real carbon fiber interior
AMG sideskirts
Custom rear wing

'96 Impala SS (Black)
17" Boyd's "Phoenix" wheels
Custom exhaust
Full custom interior(black and purple) with real carbon fiber
Custom intake

'94 Firebird Formula (Turquoise)
17" Antera "Opus One" (three spoke) wheels
Full custom interior
Vortech Supercharger
Other Misc Craziness