I found this on an old thread
“That blue connector in the first post is part of the BSD communication network. It is critical to proper function of the car. It has nothing to do with showrooms or discharging etc. More than you want to know can be found here:
http://freemaxx.de/e90forum/03_Power%20Management.pdf
The BSD faults can be traced to any of that data network's components- Alternator, IBS, oil sender, wiring, etc. ISTA-D has test modules to trace faults. You can guess at it and throw parts at it but best to do a proper diagnostics session.”