The sending unit is on the driver's side of the block, down low. Its pretty easy to find. It is round (looks like a large freeze plug) with an elongated (2" maybe) "cap" where the wire is attached.

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81 300D N/A 130K miles (Nelly)
87 300D Turbo 80K miles (The Scalded Dog...because it runs like a scalded dog)
95 e320 Wagon 104K
2003 Sprinter 47K miles.
1999 Tandem Bicycle
One beautiful low miles wife (who likes diesels) (that's my wife holding the sign)

My son and I fixing Nelly's odometer