The 77 vs 79 clock and tacho are both different units internally but believe they are wired up similarly. Tacho is separate to clock, 2 wires, does its own thing. Clock is grounded through the case/rear of tacho mounting plate with an insulated live feed off the circuit board that runs from the main harness plus. There is a ground also through the main harness, doesn't require the strap to the gauge cluster body if the harness is plugged in.