I suspect you'll end up having to hard wire it. The Grace was a pretty rare bird in its time (like 1980).
As I recall, the cable plugs in at the base of the arm and you'll have to improvise some pins or pin sockets (depending on gender of arm's connector) or may have to remove that socket from the arm to get at the bare wires for soldering.
With a simple multimeter you should have no problem confirming which wire corresponds to which in the headshell.
Good luck -- it wasn't a bad arm but a bit over-rated because it was associated originally with the Linn turntable.