One thing you want to check is if the erratic warning light behavior disappears with a fully charged battery. Put it on a trickle charger overnight and then drive to see if the issue is gone; if so, then you probably are in fact looking at a bad ground issue. My issue would go away when I charged the battery, then it would slowly return over a few hours of driving and I would occasionally have to jump start due to low battery.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280