Percolation? Heat soak? Don't drive during summer??

Fixed the buzzing coil... it was from this green wire that runs to the tach on top of the steering wheel. It was hooked up on the negative side of the coil. It runs with the black wire to the back of the tach. Im assuming it is a ground wire? But why hooked up on the coil?

Haven't started the car yet, as its to late and the kids are going to be going to bed soon. Hopefully i get home at a reasonable time tomorrow and can mess with some stuff. Does anyone know where this ground goes? Its never been hooked up on my car, but I put the green wire under it to keep it out of the way...