The 103 motor in your car can develop a leak in the passenger side corner. Been there - done that. There is also the infamous U-gasket at the front of the motor, but as you mentioned, yours is at the rear.

There has been some talk on this forum about the rear corner 103 leak and also on the Ritter-Easley list.

I had my head gasket replaced and that cured the problem.

Whether you elect to do the work yourself or have it done, consider the valve guides and valve stem seals while the head gasket is being replaced.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"