The conclusion to this little saga was reached today...
What's left of the diode once I found it buried in the harness:
Cleaned it all up:
Started pulling the harness apart to trace...
Found this buried inside:
Somebody had cut the gauge cluster lamp wire back and then added a splice to the other side. This splice ended up over at the plug for the EGR solenoid, to provide a very weak +12V through a bad diode. I guess the lamp in the dash died and rather than fix it, someone decided to splice a switched +12V onto it. Kind of a terrible way to do it...
Handy alligator clips to bypass the entire mess:
Imagine my surprise when the light started working again!
My woes weren't over as the remanufactured alternator I had purchased ended up being bad out of the box. I had to troubleshoot it for 6 hours to determine that, and then the parts store manager accused me of lying and re-tested it 3 times on their new test stand (failing all the while), then denied me a refund, but that's a story for another thread...
Fortunately everything now works with a working alternator:
Not my best work, but it's now spliced together and fixed properly:
I have to locate a diode to replace the failed one, probably a 1N1001 or something similar will be overkill. I know no one out there gives a crap about these cars, but I figure this will help someone out who is pulling their hair out over a similar charging problem that nothing seems to be working on. And to relate to Pontiac-land, this engine and charging setup is almost 100% identical to the '83-86 Pontiac Parisienne with the 307, and is identical to the '87-89 Pontiac Safari, so someone in our corner of the world may have the same problem.