From: Billo billo_at_unofficialbmw.com
Subject:  Stereo Wiring

1980's era BMWs E23?, E24, E26, E28, E30

BETWEEN HEAD UNIT AND BOOSTER AMP
  Yellow/Red - Left Front Channel
  Yellow/Black - Left Rear Channel
  Blue/Red - Right Front Channel
  Blue/Black - Right Rear Channel
  Yellow/Brown - Left Channel Ground at Amp End Only
  Blue/Brown - Right Channel Ground at Amp End Only
  Brown - All Channel Ground at Head End Only
  (note that the all of the grounds are spliced together within the wiring harness - the 
   system uses a common ground between the head and the amplifer)
  White - 12V Switched (to turn amplifier ON when head unit ON, also runs to power antenna)

BETWEEN AMP AND SPEAKERS
  Pair: Black/White and Black/Brown - Right Rear Speakers
  Pair: Grey/White and Grey/Brown - Right Front Speakers
  Pair: Black/Red and Black/Purple - Left Rear Speakers
  Pair: Grey/Red and Grey/Purple - Left Front Speakers
  (note that the wires highlighted in red above all run to the crossover side of 
   the midrange drivers, you'll want to know this to keep all your speakers in 
   phase - make sure all of your speaker (+)s go to these wires)
  (also note that the speaker circuits DO NOT use a common ground when they leave the
   booster amp)

OTHER WIRES
  Red/Black - 12V Battery Power (via 30 amp fuse in fusebox according to Bentley)
  Brown - Power Ground

Also see this article



Dale's Wiring Non-Amplified E30 Stereo:
Note: '80's era cars had external amplifier and non-external amplifier systems. Be sure to 
 identify the type you have.  All the premium stereo's had external amplifiers.

 Speaker wiring goes from Radio to Fader control in dash, then to speakers.

Radio:
  Brown: Ground  [alpine black]

  Purple/White: Accessory/Ignition 12v, main power  [alpine red]
  Red/White: Constant 12v?
  Yellow: Constant 12v, use for memory              [alpine yellow]
  Green/Red: Dash lights 12v, varies with dash lights [alpine orange]
  White: power to antenna                           [alpine blue]

  Blue: Right+ (to fader)
  Yellow: Left+ (to fader)
  Blue/Brown: Right-
  Yellow/Brown: Left-

Dash Fader Control to Speakers
  Blue/Red:   Right Front+
  Blue/Black: Right Rear+
  Yellow/Red:   Left Front+
  Yellow/Black: Left Rear+
  Yellow/Brown: Left Channel Ground
  Blue/Brown:   Right Channel Ground





1990's era BMWs, E31, E32, E34, E36

Below is the 17 pin connection color coded wires on the newer models with the BMW Connector 

X 18126

BMW CONNECTOR PIN# FUNCTION
FEMALE X18126
  YELLOW/RE 1 LF CHANNEL +
  BLUE/RED 2 RF CHANNEL +
  YELLOW/BLACK 3 LR CHANNEL +
  WHITE/BROWN 4 TELEPHONE MUTE
  VIOLET/WHITE 5 HOT in ACCY, RUN and START
  BLUE/BLACK 6 RR CHANNEL +
  YELLOW 7 I-BUS CHANNEL +
  YELLOW 7 EMPTY I-BUS
  BROWN/ORANGE 8 LF CHANNEL -
  RED/WHITE 9 HOT _at_ ALL TIMES
  BLACK/WHITE 10 VECH./SPEED SIG.
  BROWN/ORANGE 11 RF CHANNEL -
  BROWN/ORANGE 12 LR CHANNEL -
  GRAY/RED 13 ILLUMINATION
  BROWN/ORANGE 14 RR CHANNEL -
  BROWN 15 GROUND
  WHITE 16 ANTENNA CONTROL
  17 EMPTY