One postscript I wanted to add to this (for anyone who comes along later with the same problem):
I suspect the Jones Plug wasn't the problem after all, although I would not have found it without HowardR's suggestion to use alligator clip jumper wires (and chas10e's original help). When I attached the alligator clip to the alternator relay switch (to the grey/black wire on the right) I noticed that the screws holding the switch were very loose. Perhaps from the constant vibration that unit takes? So I tightened both of those screws, then connected the alligator clip to the switch, connected the other end to the alternator relay and viola, it worked. And then, when I disconnected the jumper wires, it still worked.
I initially attributed that to the jones plug connection being "cleaned" by reseating it a couple of times. But now I'm thinking it was the screws being loose on that switch which prevented that switch from closing. That seems a better explanation for why it was working one day, and then not working the next. (Better than the jones plug suddenly "loosing connection" for no apparent reason.
Anyway, that for all the help. And hope this helps someone else down the road...
Old Chicago 00-90 Unit (resized).jpg