I was exchanging emails with a fellow who has a BMW and a pretty good stereo install. On a BMW bbs site, he made a very long, detailed post that included the following information that I hadn't seen addressed here before.:

I also learned (and this is REALLY important) that the OEM head unit in our cars does not use the “standard” RCA signals prevalent in aftermarket stereos, but the much better differential output signals. This means that you “CAN’T” directly connect the outputs from the head unit into an aftermarket amp – you need a converter. There are a few recent aftermarket amps that can take the diff signal directly, but most can’t (check BEFORE you get your amp). My amp is one of those that does not, but luckily for me Peripheral makes one that works perfect with my a/d/s amp, the Vendetta 4 (also called VEN4 for about $40). I am using it and I can tell you it works!

and...

Make sure your amp can take the diff signals. I am exchanging email with another guy who tried without the VEN4 adapter and the noise was far too great once the engine was started that he had to shut down the radio. He is now in the process of buying the adapter.

I don't know if this is new, or even useful information, but on the chance that it might be, I thought I'd pass it on.

tanstaafl.