If you run the negative volt meter mount stud grounded to the metal cluster housing then you only have to run 1 wire for the 12V feed. Make sure your 12v feed stud is insulated so it doesnt ground out. I'm not well versed in the 66 plymouth cluster. Does it have a plug in IVR on the circuit board? If so, find the side of the IVR that's the 12V feed side, and attach your 12V feed wire to that side of the circuit board.
