Dave,
What you are looking for is the loop of wire which connects both L and R dash indicators on the tach. Here's my tach popped out of the dash. Notice the brown wire connecting between the 2 indicator lamps.
On your car, you simply want to remove those wires and tape them up so they don't contact anything. Then make a pigtail like the original connection except this pigtail will connect the 2 bulbs together and then connect to a chassis ground.
Seems like every year of 911 has some subtle differences in the wiring and finding the right wire diagram is crucial. I can't comment on discrepancies of wire diagram to car; I'm learning as I go as well.
