Alan,
My guess, (without physically looking at gauge cluster), would be that with the ground ending at the tach, the tach is mounted in the clusted making contact with 'bare' metal as will all of the other gauges when also mounted. Attaching the ground wire to the dash frame will accomplish the same completed ground circuit. IMO you're more of a 'Lectro Guru' than you thought. As long as the negative post on the battery is attached to the vehicle's frame, the body is making a good metal to metal contact to the frame and the dash has a metal contact to the body the circuit is complete. I have seen 18 wheelers run several thousand mile before experiencing computor module malfunctions due to painted ground terminals. Like Strodder said...
