Found what I needed on bmwtips.com:
"Locate the two Twisted pair wires (yellow=right channel, blue=left channel) from the blue connector wiring harness. We decided to cut these wires (leave enough length to reverse the process) so that any noise from the tape source is eliminated. Connect RCA(left and right) cables to these wires."
Can anyone explain why if you do this you need to install a line driver but it works fine using a cassette adapter?