Could be a short to ground. I'd start looking for chafed or worn wires. Check out all the harnesses, particularly where they go into holes drilled through sheetmetal, especially at the firewall. Sometimes a little rubbing or chafing can wear through the insulation causing a short to ground. Just my $.02.

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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