Did you follow the procedure from the shop manual? It REALLY has to be followed EXACTLY as written . I just went through a drain that turned out to be LED map lights in overhead console.
1. Make sure the bussed electrical center (BEC) and smart junction box (SJB) are accessible without turning on the interior or the underhood lights.
2. Drive the vehicle at least 5 minutes and over 48 km/h (30 mph) to turn on and activate the vehicle systems.
3. Allow the vehicle to sit with the key off for at least 40 minutes to allow the modules to time out/power down.
4. Connect a fused (10A) jumper wire between the negative battery cable and the negative battery post to prevent the modules from resetting and to catch capacitive drains.
5. Disconnect the negative battery cable from the negative battery post without breaking the connection of the jumper wire.
6. NOTE: It is very important that continuity is not broken between the negative battery cable and the negative battery post when connecting the meter. If this happens, the entire procedure must be repeated.
Connect the battery tester between the negative battery cable and the post. The meter must be capable of reading milliamps and should have a 10 amp capability.
7. NOTE: If the meter settings need to be switched or the test leads need to be moved to another jack, the jumper wire must be reinstalled to avoid breaking continuity.
Remove the jumper wire.
8. NOTE: Amperage draw varies from vehicle to vehicle depending on the equipment package. Compare to a similar vehicle for reference.
NOTE: No factory-equipped vehicle should have more than a 50 mA (0.050 amp) draw.
Note the amperage draw.
9. If the draw if found to be excessive, remove the fuses from the SJB one at a time and note the current reading. Do not reinstall the fuses until you have finished testing. To properly isolate each of the circuits, all of the fuses may need to be removed and install one fuse, note the amperage draw, then remove the fuse and install the next fuse, until all of the circuits are checked. When the current level drops to an acceptable level after removing a fuse, the circuit containing the excessive draw has been located.
10. If the current draw is still excessive, remove the fuses from the BEC one at a time and note the current draw. Do not reinstall the fuses until you have finished testing. To properly isolate each of the circuits, all of the fuses may need to be removed and install one fuse, note the amperage draw, then remove the fuse and install the next fuse, until all of the circuits are checked. When the current level drops to an acceptable level after removing a fuse, the circuit containing the excessive draw has been located.
11. Check the Wiring Diagrams for any circuits that run from the battery without passing through the BEC/SJB. If the current draw is still excessive, disconnect these circuits until the draw is found. Also, disconnect the generator electrical connections if the draw can not be located. The generator may be internally shorted, causing the current drain.
I copied the above instructions from a different Mustang forum, but they are from the service manual.