A note on balanced outputs:

Regular RCA cables carry single-ended outputs. These have the outer ring at chassis ground and the (+) center conductor carries the signal.

Balanced lines (like in an XLR connector) work differently. There is a (+) wire, a (-) wire, and a reference ground wire. The signal on the (+) wire is the same as on the (-) wire, but UPSIDE DOWN.

At the other end of a FULLY balanced signal chain, one signal is inverted and then the two are summed together. If any noise is picked up on the cable, inverting the signal puts the signals back inphase, but now makes the noise on one the exact opposite of the noise on the other. If you sum them together, the noises cancel each other out.

So the BMW is balanced (as are many stereo systems) with a twist: a LOT of DC content to the signal. Basically, the signals vary in voltage, but never cross the 0V line, so they're not true AC. We don't care about that if it didn'teventually toast your amp.

So many amps take "balanced inputs". Yes, they do, but they don't do the cancellation thing - they just float the signal ground so you can run these signals straight in. They DON'T use the (-) side of the signal.

So the important thing when working with the BMW preout is to get the DC out and get a signal your amp can use. You can do this with an LOC that has transformers in it, or with a Ground Loop Isolator (they have transformers too). Or you can use a shunt capacitor to ground (like xtant used to tell people to do with BMWs). I've done all of those. Many amps can't take a balanced input, so the LOC can give them a signal that doesn't freak them out.

Hope this helped somewhat...