Tachometer, negative wire?

hey guys, i have a question about my 82 xj550. I bought it with a original fishfinder cluster that appeared to be for a mechanical tach but was missing the cable. i took it off and im trying to install a tach and speedo from old bike barn (there is no manufacturer name) which are supposedly universal for jap bikes. Iv gotten the speedo hooked up and the light for the tach but i cannot get the tach to work. there is a green wire that the diagram says should be attached to the negative on the coil. Im not sure which wire they mean. there is a wire that provides spark then the wire that goes to ground. in between there are two small wires that go into the loom. are those the negatives im supposed to be hooking into? if so which one? i have a red and white and a blue and white.