Probably was supposed to go to the back of the head, or, the PO installed something, and just grounded it there. Any idea where the other end goes?

 

There are two black ground wires on the pass side and one on the drivers side, these go from the valve cover studs to the firewall pass side and from the firewall to the back of the head on the drivers side, these are a must be there thing.

 

Just connect it as another ground wire.