what is best way to find a battery circuit short?

Get an analog voltmeter remove the negative battery cable, attach the Black lead to the battery post, Red lead to cable end, then pull fuses one at a time, make sure there are no accessories or lights in the on position, Key off! Close the door have someone in the car, pull fuses one at a time after each fuse check meter, when you pull the fuse for the bad curcuit, the meter will drop to 1 volt or so. Check all fuses, even under the hood. If meter does not drop, remove the battery connection from the alternator, if it drops now, the diodes are bad in the alternator. Put the fuse back in were it goes as you go so you don't forget which fuse goes were! Wait several minutes after hooking up the meter before pulling fuses, test with a good battery!
https://www.2carpros.com/first_things/why_is_my_car_battery_dead_over_night.htm
GM's with Onstar can take up to 1/2 hour for all the modules to "sleep" before testing can begin!

Get an analog voltmeter remove the negative battery cable, attach the Black lead to the battery post, Red lead to cable end, then pull fuses one at a time, make sure there are no accessories or lights in the on position, Key off! Close the door have someone in the car, pull fuses one at a time after each fuse check meter, when you pull the fuse for the bad curcuit, the meter will drop to 1 volt or so. Check all fuses, even under the hood. If meter does not drop, remove the battery connection from the alternator, if it drops now, the diodes are bad in the alternator. Put the fuse back in were it goes as you go so you don't forget which fuse goes were! Wait several minutes after hooking up the meter before pulling fuses, test with a good battery!
https://www.2carpros.com/first_things/why_is_my_car_battery_dead_over_night.htm