Hello John, if this is the black/yellow/pink wire, it's the ground switch wire and goes to and comes from the horn switch on the steering wheel
Sorry, should have looked at your picture, this should be a black/yellow/pink wire to the horn switch, looks like it may have been replaced in the past with a black wire, best to check the other end at the horn switch in case someone has modified it
Where is the brown wire in the steering column going off to? That should be the ground wire to the other side of the horn switch, and, as far as i know, should also be connected to the ground (brown) connection in the column switch, perhaps someone has connected this to another ground for some reason?