According to service task 20-010 for the 602-603 engine(coolant drain), the coolant drain plug is on the passenger side of the motor towards the rear of the block underneath the exhaust. It looks identical in location to where MB puts the plug on the 103 gas motors. I'll bet a 19mm socket on a 1/2" ext. gets it out.
1991 300-SEL - Model 126
M103 - SOHC
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