[ctuna]

There are only two wires in what is considered RCA's . These have been used
forever to hook up Record Player and CD's up to receivers and amps in Home
Stereo . This means a single ended signal travels down the middle wire surrounded
by a Shield that is attached to a common ground.
This is not what comes out of the back of a BMW head Unit.
What comes out is basically a reduced speaker out when programmed for Hi Fi .
That means neither wire is common ground. There is no third wire when you
hook this system up . Also because the signal in each wire is of equal an opposite
value it provide a noise canceling function.
Read OP amp theory and differential Inputs if you want to understand the
theory.
The better amplifiers will detect the non common ground and convert to dual ended
differential Inputs.
Because it has twisted wires also known as a twisted pair this keeps the noise down.
You should Run a differential RCA Cable on all long runs to keep noise down.