I'd be tempted to tighten each connection a little. Maybe a 1/4 turn if they don't seem too tight now.

On my'87 190D, I removed the aux water pump and replaced it with a brass connector. The heater worked fine and seal leak was gone. I also had a leak from the heater control valve. It is located betweeen the fire walls near the battery. It is an easy relatively inexpensive repair. On my car I was seeing coolant leak out near the transmission, also. I, too, feared a heater core issue, but it turned out to be much simpler.

Good luck. YMMV

__________________
Tom Savage
Vienna, WV
1984 300D Euro
1995 S320