Connector A where the harness connects to the cluster in the middle. But thinking about it some more, it would be easier to run a wire directly from the sender tip on the head to the temp gauge. As you know, the temp gauge has three screws that attach it to the cluster chassis and printed circuit foil. One is 12V, one is ground, and one is the variable signal from the sender. That's the one to connect your test wire to. You can figure it out by following the three foil traces attached to the meter screws, then verify it with your continuity checker. Be sure to have your original sender wire disconnected. This way you eliminate all the wiring and connectors from the sensor to the temp gauge. Stick the cluster back in with the new test wire, and see if the gauge indicates correctly. If it does, then there's a wiring problem between the sender and the gauge. Make sense?