My 2004 BMW E61 had an odd optional extra on board: a proper car telephone in the armrest.
With no intention to get a SIM for it and just having seen a commercial kit for auxiliary input jacks (Belkin AirCast?) I startet to build my own one. After all the headset for my Motorola Milestone had the same type of TRRS-plug (tip-ring-ring-sleeve).
It was straightforward: Get connectors, get an appropriate cable, solder everything together. I decided to keep it standard with RCA/Cinch-plugs and an audio jack for the microphone. An old hands-free mic could be scratched up and directly received the proper plug.
Because of the standard connectors I could test everything in the living room with my hi-fi system. It worked, so the next step was to get an “Aux” cable from eBay to extract the signal from the Quadlock connector of the “CCC” unit of the 545i – and also to hack the right parameters into the Car Communication Computer (navigation system “Professional”) to have it activate the line-in.
(Original post: http://www.metalforum-owl.de/viewtopic.php?f=21&t=138&start=80#p39508, July 5th 2011)