by "shared ground" i presume you are referring to the 3 bolts set into the frame behind the instrument panel with about 6000 wires coming into them?
i take it the solution to the problem would be to carefully take apart each bolt and clean the terminals?
i have been thinking it's a ground, but those common/shared ground bolts "look" fine
a case of looks being deceiving?
i'll pull them apart
thx
will advise

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1985 MB 300SD
1982 MB 300SD
1977 Jawa 634 California w/ sidecar