When I back-fit a tachometer to my '85 300TD, I just ran all new wiring from the tach amp to the tachometer and to a fuse for power. Left a lot more hair on my head.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC