Thanks to Euro1011, here are the complete pinouts (I had the numbering in reverse order):

1: Window Antenna Coil Amp.
2: Ign/Switched +12V
3: Steering Wheel Controls +ive.
4: Multiplex Integrated Control Unit
5: RR +ive
6: LR +ive
7: RF +ive
8: LF +ive
9: Illum./Dash Lights
10: +12V
11: Steering Wheel Controls Ground
12: Nav. Unit
13: Nav. Unit
14: Nav. Unit
15: RR -ive
16: LR -ive
17: RF -ive
18: LF -ive
19: Dimmer
20: Ground