4 amps sounds like a lot of current to flow through a diesel-soaked wire laying against something grounded. Based upon your prior investigations with pulling all the fuses, it would have to be in an unprotected circuit as well.

Do you have any aftermarket electronics that may have been installed with a direct connection to the positive battery terminal?

Do you have the wiring diagrams for this car? Are there circuits that have fuses or breakers that you haven't yet eliminated?

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1987 W201 190D