There is a drain plug on the side of the block. Drain and flush with water first.

I've replaced the heater core on a w201 body and a w115 body. Neither were easy as it means taking the whole dashboard out. Makes sure it's really yhe heater core as it could be a leak in a line or at the connection to the heater core. Try to find where the leak is. Look for coolant under the floor mats or under the car.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)