Leaky heater core, if it was my car I would first put in alumseal and try to seal the heater core with that, turn the heater up to high, fan on low and add this to the radiator being careful not to open under pressure or when hot as not to burn yourself, if that does not stop the leak then you have to change the heater core

__________________
Jerry

2004 SL500 Black 55K Miles
2002 s500 Black 90K Miles
1986 300E Gold 233K Miles
1966 Twin Cessna 320D
1989 Jeep Cherokee 233K Miles
1994 Ford Van E150 399K Miles