On my 500SEL (I think they are the same for tach and clock) the single wire connector is battery power for the clock and the 2 pin plug is ignition power and tach signal (TD) for the tach. The ground for the tach and clock is through a flat buss bar that goes from the tach/clock circuit board to the speedometer and then to G102 behind the instrument panel.
Hope this helps.

PaulM

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84 500 SEL (307,xxx miles)