My right rear door woofer kept going silent and it was driving me nuts! Last night, it finally gave a clue as to the issue. I had the volume cranked, and it sounded like the woofer was blown. I was sure it was time for a $120 speaker replacement from BMW. Easy enough.

Then, I open the door, and take a listen... it's playing completely normally!

Finally, I decided to take a peek into the door harness, and found one of the speaker wires was frayed and had overheated the sheath causing it to be discolored from the heat!

Cut out the bad part, soldered it back together and wrapped in BMW's special tape, recoded the DSP amp. Problem solved!