For the M103, on the horizontal surface of the head near the #1 and #2 injectors there are a couple of 13mm bolt heads. Some cars have one bolt head and one sensor. Remove a bolt then pour coolant into the reservoir. Keep filling until coolant leaks out of the hole. Cap the hole, top off the reservoir and you should be good to go. Check the reservoir over the next few trips.

Sixto
87 300SDL