found this on bmw-planet.net http://www.bmw-planet.net/diagrams/release/en/e85/index.htm
A18 Amplifier plug
X10266 42-pin, black Plug connector Amplifier
Pin assignments at plug connector X10266
Pin/ Type / Description / connection
1 A Signal, rear left speaker - Subwoofer, left
2 A Signal, rear left speaker + Subwoofer, left
3 A Signal, front left woofer - Woofer, front left
4 A Signal, front left woofer + Woofer, front left
5 E Terminal 30 Fuse F102
6 Not used
7 Not used
8 Not used
9 E Signal, AF radio, front right + Radio
10 A Aerial switching signal Aerial diversity
11 A Signal, front right tweeter + Tweeter, front right
12 A Signal, front right tweeter - Tweeter, front right
13 A Signal, rear right mid-range speaker + Mid-range speaker, rear right
14 Not used
15 Not used
16 Not used
17 E Signal, AF radio, front right - Radio
18 E/A K-bus signal K-bus connector
19 Not used
20 Not used
21 A Signal, rear right mid-range speaker - Mid-range speaker, rear right
22 A Signal, front right woofer - Woofer, front right
23 A Signal, front right woofer + Woofer, front right
24 A Signal, rear right speaker - Subwoofer, right
25 A Signal, rear right speaker + Subwoofer, right
26 M Ground Ground point
27 Not used
28 E Mute Cellular telephone
29 E Signal, telephone AF + Cellular telephone
30 E Signal, AF radio, front left - Radio
31 A Signal, rear left mid-range speaker + Mid-range speaker, rear left
32 A Signal, rear left mid-range speaker - Mid-range speaker, rear left
33 A Signal, front left tweeter + Tweeter, front left
34 A Signal, front left tweeter - Tweeter, front left
35 Not used
36 Not used
37 E Signal, terminal 50 via vehicle immoblizer Cellular telephone
38 E Signal, terminal 50 via vehicle immoblizer Radio
39 A Signal, front left mid-range speaker - Mid-range speaker, front left
40 A Signal, front left mid-range speaker + Mid-range speaker, front left
41 A Signal, front right mid-range speaker - Mid-range speaker, front right
42 A Signal, front right mid-range speaker + Mid-range speaker, front right