Frank,
This might sound stupid but one way would be to go through a process of elimination. Your voltmeter probably can also serve as a continuity tester.
Turn off your ignition key.
Disconnect your ground cable from the battery.
Connect your continuity tester between the negative post of the battery and your battery cable end.
If you get a continuity reading, you have a short somewhere.
Now, start disconnecting electrical leads one at a time. If/when you loose continuity, you found the source of the short.
Start with the leads at your fuse box (start removing fuses).
Then disconnect the generator leads to the voltage regulator. (A wild guess would be that the generator has not been polarized and current is running "backwards" when you shut off the ignition).
Many years ago, I used this technique on a VW and found a short in the electric clock, which normally runs all the time but was the only device drawing current with the ignition off. A short in the non-working clock was draining the battery very slowly.
Tim
