I'm not familiar with this car but I did have annoying coolant leak on my 190D that turned out to be the heater control valve. It was a slow leak that ended up in a drain tube from the compartment where the valve is located. I had visions of freeze plugs, heater core, etc. Turned out to be a really simple $25 fix. I mention this to encourage you to look for possibilities outside the standard engine "box". Good luck.
1984 300D Euro