Originally the car had BMW Business CD with 10 speakers and an amp (Alpine) in the trunk and no DSP. I replaced the speakers in front with separates and also upgraded the amp with a Ground Zero one.

I used the outputs using the wires indicated by a page in bmwtips labeled E39 speaker color codes. Connection was made soldering the original wires to RCA plugs. As there are several same colour wires I assumed the pairs were always + and - of the same output (in other words I did not measure from the plug).

Connections:
brown-orange = left front -
blue-red = left front +

yellow-red = right front +
brown-orange = right front -

When testing the system I found the following problem using the front outputs:

- when right and left RCAs are connected to the amp I have the right and left channel test signals in both channels all the time and using the balance adjustment only marginally turns the sound level down when not in center.

- if only one RCA is connected to the amp it seems to work OK, sound only from one channel and balance works

So I made a test. I soldered also the rear outputs to RCAs. When I plug them in the amp all works well! Test signal from only one channel, balance adjustment etc. (Only fader works the opposite way of course)

connections:
brown-orange = right rear -
blue-black = right rear +

yellow-black = left rear +
brown-orange = left rear -

For now I only have the front speakes but will also install rear speakers so I will need all four head unit outputs to work. And the amp has 4 channels (accepts only RCA input).

I have tested that the front outputs are not in short circuit so what is going wrong here?

- do I have wrong colour codes (car is 2002 ym. e39, built late 2001)
- is there a difference of some sort between the front outputs and rear outputs
- what should I test or change

P.S. In another thread a similar problem existed. Solution was to install a processor but I would like to understand WHY one is needed and - more importantly - if there are other (cheaper) alternatives.