According to my 1998 328i Bentley manual wiring diagram, here's the breakdown:
red/white = hot at all times
brown = ground
white = antenna amplifier
violet/white = hot in ACCY, run and start
blue/brown = right front high range speaker (this also branches to rt fr low range)
blue/grey = right rear speaker
blue/violet = right rear speaker
yellow/brown = left front high range speaker (this also branches to lt fr low range)
yellow/grey = left rear speaker
yellow/red = left front low range speaker ) this branches to left front high range)
blue/red = right front high range speaker
I don't see a yellow with blue but there is a yellow/violet which = left rear speaker.
Also, your grey/white line is either for the interior light system or amplifier.
I dont know what year and model BMW you have