I had the same issue with a 2001 Mustang. Here is what I found out in the hopes it will help some one out:
Battery would be dead after a few days if car was not driven. Checked current from battery with car completely turned off. Over 300mA current. Started pulling fuses under the hood. 40A Maxi fuse number 6 caused the current draw to drop to a few milliamps. Problem was shorted output transistors in the Mach sound system bass amp in the trunk (right side). Found it by pulling connectors, one at a time. Seems the amps get standby power to keep the electronics “warm” to avoid a pop when turned on. Found replacement transistors at Digi-Key electronics and fixed it up. All works fine now and car can sit for two weeks and start just fine.
Now for the data that will help you troubleshoot:
When operating normally, remove fuse F6. Put your meter in place of that fuse and set on Milliamps DC. When you close the doors with the car off the meter will read over 200mA (I have an LED dome lamp - yours may vary) until the dome light shuts off. Then the meter will read about 112mA for about 40 minutes. After that the PCM goes into standby and the meter will read about 12mA steady state.
Anything else, and you have something causing a parasitic draw on the battery.
BTW, I mentioned this repair to my daughter who used to drive the car and she said “Oh, yeah. That was Bone Thugs and Harmony”