This may help:
The 'hidden wire' has a red color.
Note that you're looking at the female connector. The contacts in the picture are grouped as:
A6 A5 A4 A3 A2 A1
A12 A11 A10 A9 A8 A7
And this is what they mean:
A1 - Supply
A2 - Switched supply
A3 - Rheostat signal (Signal from the central electronic module (CEM). An increased pulse ratio gives greater luminance. (If there is no rheostat signal, there will be maximum luminance in the audio module)
A4 - Speed signal (Pulsed signal from the combined instrument panel. Frequency increases with increase in speed.)
A5 - Back-up (reversing) lamp switch, input signal (The back-up (reversing) lamp switch must be activated when reverse gear is engaged.)
A6 - Carphone mute
A7 - n.c.
A8 - Power ground
A9 - Remote control signal ON/OFF (Remote control:antenna amplifier (V40), power antenna (S40), Bass speaker (V40) (Integrated power stage).)
A10 - n.c.
A11 - n.c.
A12 - Power ground