I have a cable behind my head unit thats really bugging me. It has four cables, Red, Red, Blue and Black. I have tested with a meter and one cable is the reverse signal to put the head unit rear camera on. Another is the brake signal, to stop you watching videos while driving, the other two seem to be and earth and 12v, but I cant see any point for them as the head unit feed is on a different plug. Im kinda thinking its some sort of signal voltage but i can see any voltage change. 

 

Does anybody have an idea what these two wires are actually for because its starting to drive me mad

The are a few taped up connectors hidden in the dash, there were several headunit types, that's all.

1 hour ago, Chris.ac said:

The are a few taped up connectors hidden in the dash, there were several headunit types, that's all.

 This one was not taped up and had been used. As I said it has the reverse line and the brake line in the same connector. I cant see why it would have another 12v power and earth in that cable when the unit would draw power from the main power connector which is why it is bugging me so much

Its for the low spec (4 speaker, no external amplifier) headunit. 

Red 12v is ignition on sense, black is actually speed signal (to adjust the volume when speed increases, u should see it change voltage if in motion) and you rightly state the other red and blue are reverse and parking brake 

 

The connector is referred to as F5 in the ODO wiring schematic.

 

There would have been an additional 12v battery (for headunit memory/clock) and 12v supply from the radio fuse provided via a different connector

 

ODO.pdf

 

Here is the wiring for the low spec headunit.

 

Its like i said, there are a few connectors for different headunit specs, the fact it was not hidden away suggests a previous owner changed headunit, or was using it for some aftermarket purpose. A handy switched live wire?