Thanks for all of this practical advice.
I think my course of action will be that I will leave it "as is" for a month or so, but run through Richie's checklist.
Then, when I have calmed down, I will have to remove the whole switch to get the motor cover off (since this is one of only two switches where I left the motor cover screwed down when I mounted the switch), and see what is inside.
If I can't really see anything, then I'm going to try using a barrel connector to go over the split-4 pin. (I just saw one of these last week when I replaced a 6 volt tail light on my tractor, and the light came with one of these barrel connectors attached.)
If that doesn't work, then I'm going to permanently solder a 6 inch wire to it, and wrap it in electric tape to seal it up.
When I remove the motor cover, if I see anything mechanically wrong, I will post a picture of it here, as a warning to others of what can happen.
My current "theory" is this: I always had trouble with this particular split-pin, because the original plug it came with was too loose and it was giving me a spotty connection. So, I very gently "spread" the four parts of the pin with a razor blade, to open the parts up just a bit, and put the plug back on. This lasted for a few months, but then I started having some spotty current flow again.
So, I threw the original plug away, and replaced it with a new one. The new plug seemed to have a noticeably smaller hole in the end, and fit much tighter. This seemed to fix the electrical problem.
But, I had to pull that plug off, in order to fit the track bed under the motor housing.
I think that when I shoved it back on, the hole in the end of the plug was not totally lined up at a 90 degree angle with the four-split pin. I think that the hole captured three of the pin prongs into the plug, but the plug housing got "underneath" one of the pin prongs, so that when I pushed the plug in, it peeled that pin prong up and away from the rest of the pin like a piece of banana peel. That "one fourth" pin section then touched the metal housing or an end wire inside of the switch, and carried the constant voltage current straight into the track.
Just my theory. I'll let you know when I open up the housing.