2000 Mustang 3.8L battery drain

IMO the problem is unlikely to be in a fuse out of the the battery junction box (BJB). The BJB is full of "always on" circuits that feed other circuits. Your time is better spent looking at the Central Junction box (CJB).

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/for...r-swap-wont-fire-please-help.html#post2669271

Here's some information on how to perform a "voltage drop" test. Use the voltage drop method to "infer" the current through each CJB=F2 fuse by measuring the voltage drop across each fuse. The amount of voltage drop is less for larger fuses. There are tables for this. With everything off record the voltage drop across each CJB fuse. Use the fuse number scheme in the fuse panel schedule. Once the fuse is known the next step is to follow down what the fuse supplies.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

Note, if looking for an educated guess look first at the radio and/or the amp.

I have the same problem,I found a ground wire that is bolted to the fuse box under dash,it is a ground for my stereo sub woofer,factory system.i have not found a fix yet..since I unbolted it,my battery no longer drains out over night.