The best way to find a leak like this..ie..one that's not glaringly obvious is to connect a cooling system pressure tester.
With the car level and engine cool, remove expansion tank cap and connect adapter that comes with the tester. Auto Zone and other mass-marketers rent these gadgets.
Pump up to about 15-17 lbs. - wait a bit, then look underneath for where the coolant is landing on the ground, then trace back upwards. The possibilities on 103/104 motors are many. This device will tell you very quickly where it's coming from. Your leak could be coming from the vicinity of the heater control valve - in between the 2 firewalls - passenger side.
If your car has the large encapsulation panel underneath, you'll want to remove this first so the coolant falls down to the ground. If you leave the panel on, the coolant will land on it vs. the ground. Seeing where it lands makes tracing back much easier.
1991 300-SEL - Model 126
M103 - SOHC
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