I have same issue with my 03 E53! I removed factory subwoofer and discovered water on the carpet area also where my DSP amplifier is there is water. Both Rear qtr panels are sopping wet ( I dried as best as I could with towels) i did wash the truck and it rained on xmas day so that may have been the culprit. I discovered it on accident as I was trying to change a brake bulb i got a message in my cluster to check rear brake lights! I asked a friend who has a friend that use to work at BMW as a technician so hopefully he knows exactly what it is.