I have read here that the problem is sometimes with the ignition switch. In this case, you would likely get around 12v at the plugs, but the fan would still fail to operate reliably because the switch is unable to handle the load (meter would drop to Zero volts under load if you were still measuring).

A 2nd hand solution, but perhaps worth checking out.
Best of luck.

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1986 300E 5-Speed 240k mi.