"If I connect a voltmeter between the post and Lead A as I have identified it I get a voltage drop of twelve volts and I am interpreting this as a short."

Stop right there.

Repeat the test with an ordinary automotive (12V) light bulb.

If the bulb lights, you probably do have a short circuit worth hunting. If it doesn't, you need to repeat the test with an ammeter, not a voltmeter. Circuits to retain memory in the engine management computer and radio, in addition to the clock, draw enough current normally to indicate 5 to 10 milliamps on an ammeter.

Using a voltmeter as you did was the method taught in the 60's to mechanics for finding a parasitic load draining a battery. Since then, electronics in our cars have forced us to be updated in our methods of sleuthing for short circuits.
--
Art Benstein near Baltimore

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