I may have misread your post. If you want to use a bulb with 1 contact you need a metal socket with 1 central contact or a plastic socket with a central power contact and a second wire that contacts the side of the bulb. If you use a metal socket just use your present POWER wire and make sure the tach housing is grounded to the car either through the dash or a separate wire to the body. You won't need the ground wires unless you want to use them to ground the housing. If you get a plastic socket with a central power wire and the side ground wire just connect it the same as your present sockets.
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