You are 100% right about using a meter to complete the circuit when testing current. Sorry.

It can’t be (well, 99.999999%+) the battery.

Either a connection or the alternator is slowly failing. Check the small wires as well.

Found this;

After firing the engine up, you should get over 14.2 volts. If you got 14.2 or more volts, the alternator is charging fine. If it’s still showing in the 12’s, it’s time to replace the alternator. If it’s producing a voltage level in the 13’s, go ahead and take it into a parts store and have it tested. It’s probably bad.
Now, assuming that the alternator was producing over 14 volts, go ahead and turn on the headlights, air conditioner, radio, and any other accessory you can think of, and run the same test all over again. If the voltage dropped back into the 12’s, it’s a very strong indication that your 4Runner’s alternator is bad. It’s still worth having it tested, but be prepared to replace it.

So maybe try again with as much of a load on the alternator as possible and see what you get for voltage.

Derek V