the ground for the whole board comes in on the half moon connector. Most of the stuff on the board is connected to this through the printed circuits on the inside side of the board. The tach, panel lights, and blinker printed circuit grounds are not directly connected to this, I assume because of not being able to cross other circuits on the board. Thats why there is a jumper wire. It goes from printed circuit to printed circuit. I would guess that when you turn on your panel lamps they are grounding through your blinkers and the tach.
You will have to take the instrument panel out and then the the circuit board out of it and check printed circuits and that wire. It probably just needs to be resoldered to the board. Or you could go to the junkyard and get another cluster.