Stop Leak? Leaking heater core?
Replacing the heater core is a very lengthy and thus costly job (20 hrs or so). MBZ put a lot of faulty design heater cores in their cars and lots have cracked and leaked. (This NEVER happens in U.S. 'junk'!)
The BEST stop leak is called "AlumaSeal', is a very fine aluminum colored powder that comes in aluminum colored plastic tube (1" dia x 3 " tall) It DOES NOT plug up any coolant passages what-so-ever, only very fine cracks and pinholes.
At a cost of about $3, it's worth a good try before laying out $2000+ for heater core replacement. No need to drain anything other than enough coolant to get it flushed into the circulation coolant.