i left the plug that was in the back of the tach disconnected. the 'mystery wire' was indeed the speed sensor for the cruise control.

sort of irrelevant since i have done away with cruise on both of my 300D's :-)

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car