Would having the circular connection plugged in make a difference? If the vehicle didn't originally have the tachometer I don't see where that plug would make a difference, i.e. I don't think there are any connection to the tach/clock portion of the gauge cluster.

I have the original gauge cluster in the car with everything plugged in. The new/donor tachometer is on the seat next to me. As described before I have everything wired as follows:

1. The white wire from the RPM sensor to the top 'W' pin on the tach (for testing i'm just using a long wire coming from the engine through the driver window)

2. The brown wire on the RPM sensor to ground (this is going from the tach to grounding cluster behind the gauge cluster)

3. 12v power to the '+' pin on the tach (another longish wire attached to the #12 fuse holder coming through the drivers window to the tach)

I'll try a few more things tomorrow. I'm not sure it would make a difference but I will run power to the 12v clock spade on the combo tach/clock.