got around to pulling the dash out on my 69 cuda today. my tach has 1 blue wire that comes out of the harness and 1 gray that goes to the negative on the coil. I hooked the positive from the battery charger where the blue wire goes and when I touched the negative from the charger where the gray goes, sparks flew. it only touched for a split second so I hope no damage was done to the tach.