If I remember right, there was some tech information that alluded to early 1994 cars not having the switch in the cubby, but it would be added later as a running change. I don't think that ever happened. I know the wires were in the harness for 1995-96 as well, but they had no switch either.
The other device that can program key fob remotes for the XJS was the Jaguar WDS; and the chances of finding a dealer with a working WDS is probably better than finding one with a working PDU.
Good luck!