I am installing a tach in a 1982 FJ40. I need to tap the negative side of the coil, but there are no markings on the coil and the wires are both same color (all original wiring and coil, I think). The wires all seem to go to the igniter on top. Both post on the coil complete a circuit to ground on the chassis with no power on at the switch. How else might I see which is negative?