I'm pretty sure the proper grounds are in the normal connector to the unit. It is common for the ground to fail in the cluster itself. The individual units of the cluster carry the ground between themselves by being screwed together. For some reason this sometimes don't get it and I imagine many have added extra ground wires.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician