Could be the o-ring where the trans. dip-stick tube attaches to the transmission housing. I had that once.....resulted in consistent slow leak, with small puddle every morning. The dip stick tube has an o-ring on the end of the tube....once inserted back into the trans. housing, there is a plate with one bolt that holds the thing in there (in addition to the upper dipstick tube mount which must be detached and re-attached). Very tight quarters with other things in the way make this one of those "very simple but real PIA" type repairs.

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)