Hmmm just a shot in the dark but I would check grounds to the offending ckt i.e. if it is in the illumination, and you think the tail lights have been changed that might be a good place to look.
Pulling the fuse for the BCM would be the same as turning off the lights. The BCM controls all body electrics. The light switch, or the auto lights on sensor, are just inputs to the BCM, the input goes ground, the BCM sends 12v to the light circuit.
If it's only one door speaker, I'd look in the area of the speaker, door grounds, fender grounds, etc.