Let's figure this one out. Keith - since you don't have (1) fog lights, do you have (2) convenience control, (3) deluxe belts, (4) A/C, and/or (5) tilt? These are items that might put relays, breakers, and such on the "Z" plate attached to the wiper motor mounting bracket.
Richard - do you know the specifics behind the application?
No, I'm not sure as to "why" the ground wire is there at this time. I've been wondering this also. Mine has a Ford engineering number on it so possible is a result one of the other options you mentioned. I know any car with deluxe seat belts, the timer mounts on the wiper motor bracket but as you may see, the wiper motor itself is grounded through a brass wavey strap to the black wiper bracket so in my opinion, this might be a ground source to one of these other options?
I'll try to look at what it plugs in to. I have looked at my "disassembly pictures" and so far, they have been no help on this connection. My pictures of this area before cleaning things up has it accurately as I pictured after making the splice in the wire. (pictured above) but the pictures I took with the motor mounted under the dash, do not help at all.
Strange as it may seem, I have another screw (out of sight in above picture) on the other side of the motor that I reinstalled it's hole at the time I disassembled the whole car. I think THAT screw pertains not as a ground but maybe for the AC option, possibly that other screw holds the LH vent ductwork away fron interfering with the wiper linkages?