Do you know where or how the fuel got on the motor mount? The most common leak is right on top of the IP. It really shouldn't be to difficult to see if that was the case. If there is is diesel on your driver side motor mount it's definitely cominng from above it. Also check all of the hoses in and around the area.

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1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k