I put a red pointer toward the fitting that seems to be leaking. See pic below. I assume it is not an electrical connector as it has a fule line going to it. Is there a small rubber o-ring inside that fitting?

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1996 E300D
1996 E320 Wagon 4MATIC
2003 Jetta TDI- GLS, 5-spd
1997 Passat TDI
2002 F350 Crew cab 4x4 Powerstroke
Jacobsen HR 15' cut ZTR w/ 80 HP Perkins diesel
Hustler 6' front deck with 28HP Kubota diesel