I have a 1990 190E 2.6. Same thing. Leak is coming from somewhere by the firewall on the driver's side. I checked the hose-to-core connection and found no dampness or residue. Just like DAVEK, there is no coolant inside the car.
Can anybody out there help on how to access the core. Do you come in from under the dash? Specifics would help tremendously. Thanks in advance.