Found it:

Remove key from ignition.
Pop the hood.
Verify all lights are off, any accessories plugged into the cigarette lighter are unplugged.
Close doors and trunk.
Raise hood.
Disconnect the battery ground cable and insert a digital multi meter in series with the battery post and ground cable, configure the meter for current measurement.
.... You should see current flow of at least a fraction of an amp up to 2 amps.
... If you see NO current, you have the meter configured wrong, or the internal meter fuse is blown.
... Once you get a current reading, WAIT THREE MINUTES.
(It takes time, upon power up, for all modules to hibernate, the reading you get is meaningless until all have hibernated - two minutes may be enough, but make it three minutes.)

After three minutes, the current reading must be .055amp (55 milliamps) or less. Mine is 19mA or so (about .019 amp)

If you have a lot of current, start pulling fuses in the big box next to the battery. SOMETIMES, pulling a fuse will make the current go UP - that is because you "wook up" a module somewhere, wait 3 minutes again. After three minutes, the current you see is the real current, if it is over 55mA, keep going.

Eventually, you will pull a fuse that makes the current go down. IF you can't find a fuse that does that, go to the little cigarette-box-sized box next to the battery, open it.
Inside are three bolt-in fuses. One 50amp fuse powers the engine fan. One 50amp fuse powers the brake controller. The 200 amp fuse powers the entire car.
Start pulling the 50's. If neither does it, pull the 200 amp. If that 200 amp one does it, we have work to do.

If the 200amp does not do it, yank the big fat red cable from the back of the alternator. If THAT does it, have the alternator rebuilt.

THAT is how you diagnose this.