1975-1977 Corvette Tach Connector Wire Location
When testing the tach before you pull it from the dash always test first. Test from the tach filter connection to the signal connection as shown in the picture below. (A to B). This is your signal wire and if it’s broken then your tach won’t work.
This connector can be un-plugged without removing the dash from the car.
You should also test for power with the key on and use a different ground source, if you have power then test again using the ground in the connector. This will verify there is not a ground issue in the circuit.