IF ITS ANTI-FREEZE = HEATER CORE. IF ITS WATER EITHER A WEATHER STRIPING FAILED OR THE A/C DRAIN TUBE IS CLOGGED UP.
The Mercedes 250's Had an issue with the drains for the sunroof plugging up. The over flow water ends up on the floorboards. If your car has a sunroof, Clearing the drains will do the trick.