Dirk - it has taken awhile but I think I know what that wire is for, it should be a grey wire but it is for the tach. The picture you posted is pretty blurry but I went out and looked at my 70 Challenger and then I back tracked the wiring diagrams. This would explain why it has a ring terminal on it, goes to the negative side of the coil.
I dont know why the diagrams are so confusing and at times incorrect but they are; therefore it takes some investigative work to figure it out. Do you have a rallye dash? If so, if you can get to the tach, there should be 2 wires, one of them is supposed to be a dark blue wire and the other a grey wire, but in you case it seems its tan but might be grey inside.