I have an old beater of a minivan, a 2000 Ford Windstar. It's mainly sitting around these days, semi-retired. Last week it got driven and the battery light was on the whole time. I measured voltages and found that the charging system was not operating.
Here comes my mistake:
I got out the manual (Chilton's, not the best but it's what I have) and looked at the wiring diagram. Even though the manual theoretically covered 1995 to 2003, it showed wiring for only through 1998. I checked voltages assuming the diagram was close enough (there's the mistake), and determined that the alternator must have failed. $190 later, new alternator installed (a very easy job), and voila, no improvement.
I did some googling and eventually found the CORRECT wiring diagram for a 2000, and found that the powertrain control module now has a hand in controlling the charging system. I pulled the cowl off to get to the module and right there in plain sight, the wire of the correct color was nicked and had, clearly over the course of many years, corroded all the way through.
Luckily the wire in question was easily accessible on the outside of the bundle, so I was able to strip the ends and do a quick soldering job.
Some waterproof tape and 10 minutes of car re-assembly and everything is operational again. The $190 is my punishment for not being thorough in my research. The new alternator is probably worth more than the car at this point.
Here are some keywords in case someone else has a similar problem and is Googling for the answer: Ford Windstar battery not charging even with new alternator/generator