[efunroom]

Interesting findings:

The outputs of the head unit are isolated from the vehicle ground. Every signal in and out of that amp is internally bypassed via a 0.22uF cap to ground -- the ground leads are not tied to chassis ground, they are floating.

The E46 head does not put out ground referenced outputs - it supplies differential signal pairs that are terminated in an impedance matching network that feed an Op Amp (in the factory amp input circuitry) that is set up to remove noise induced into the lines on the way from the head to the amp in the rear. This is very good engineering - it's easier to remove the noise than to keep it out - but it does not utilize "standard" unbalanced line levels and it is not unbalanced speaker level signals

From : http://www.e46fanatics.com/faq/stereosaga.html