Originally Posted by alexwhittemore
If you have coaxially constructed RCAs, then yes, they're theoretically shielded, but they're not quite the same (and not quite as good) as the shielded twisted pair that I suggested. That said, if you construct the interconnect right, it's PROBABLY going to work just fine - even people who don't use shielded (or even twisted) wire at all don't tend to relay that their conversation partners get annoyed by noise. Not that that means there's no problem.
Presuming you DO have coax RCAs (and not simply twisted ones), you'll want to use the core from cable 1 as Mic+, the core from cable 2 as Mic-, then you'll want to solder the shields from both cables together and connect that to pin 23 on the MULF side (pretty sure it's 23, anyway, read one of the other guide posts floating around, maybe even in this thread, don't remember where I am).
That will probably have performance pretty similar to using the normal shielded twisted wire I linked, although I think it theoretically doesn't have as high a noise rejection performance. The other problem is impedance matching, which could matter over a long run like this, but at such low (audio) frequencies, I'd be surprised if it made a difference. Plus, it's not like I measured or did any matching on the mic or wire I picked, so whatever.
it's pin 21 from the circuit diagram in the BMW bluetooth retrofit guide for cars without phone prep. see attachment.