"disconnected the battery ground, and put a meter between the battery and the ground cable. I got a consistent -12.5 or minus what ever the battery voltage is."

You should have the meter set at DC amps, not volts. That way the meter detects the flow of electrons (current) through it that are going to the appliance using those electrons. Ie. the battery is being drained by something.
--
1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

Hi there in the new year!

The description you posted about measuring from several grounding cable with a meter is a little confusing to me. I can see how you would get those readings but that's not how you use the meter to find a drain.
You need to use the ampere settings "in series" like another method I will suggest in a moment.

Your statement about thinking its a starter wiring issue makes more sense. This is so in that the alternator puts power into the system using the starter circuit.
A heavy wire runs from the alternator to the starter terminal and then up to the battery on the battery's wire from the positive post.

First you need to disconnect the battery's negative cable so the car is completely dead.

With everything off in the car and the door shuts so there is no interior light or trunk lights on.

Put a known good tail light bulb brass side and soldered tip terminal between the negative post on the battery and the grounding cable.

If the light burns "on" to get bright, you definetly have a drain.

You then briefly, disconnect that setup so you can safely disconnect the large red wire from back of the alternator and isolate it from touching anything.

Now, Hook up the original test light bulb setup between the ground cable. If you do not get a light after this you have a short that is in the alternator. If it's stays on you got to look elsewhere until the light goes out.

If a (one of many) diode has shorted to ground inside it, then you have found the reason for a battery to be drained overnight.
Depending upon how many diodes go bad it can literally turn the sides of your battery warm.
Has that ever happen?
Has the car ever been jumped with any jumper cables reversed?

Anyway, If the light stays on, you will have to keep looking elsewhere until the light does goes out.
If you use a meter it will tell you how much of a drain it is in amperes or parts of an ampere.
Hint is, you want less than a half ampere.
Like .5 is probably way too much unless a car alarm is on and I mean on!
You want to see like .05 or 50 milliamperes.

Trust me, you got to watch them decimal points in every trade!
Phil