Thank you for these suggestions- some of which I have already tried.
The small red wire on the alternator only shows approx. 1.5 volts on it- with
the engine running, while the large red wire (to the starter) shows a steady 12.3 volts; neither of these terminal's voltages increases when the engine is accelerated.
It is very difficult to trace the (small) red wire on the alternator- as it immediately is shrouded in a larger bundle of insulation; but I'll try to find the other end where it eventually emerges, and do a continuity check.
Upon pulling the IP, I did notice the unconnected / loose red wire; but saw no
empty terminal on the back of the unit- to which it might belong... figuring it might be for an uninstalled component. Thanks for that confirmation.
Besides all of the above- what I don't understand here is the relationship to the overdrive switch