There are two types of interface:

One is to grab the output from the HU to the amp. With BMWs, the signal often has a DC component to it that needs to be removed - and I think some of them are balanced three-wire and not single-ended RCA-style two-wire. Soundgate has one that does this - it's the LOCPREB, I believe.

The other is to grab the output from the amp (either internal or external) to the speakers and lower the amplitude in order to be able to take that into an aftermarket amp. There are many that do this, but it's a problematic process (see my post about the X5 analysis). If you want to add the signals back together you need transformer islation usually, which means AudioLink or Soundgate LOCHVA or Navone L7.

But you first need to decide what kind of interface you want to, and can, do, before you can decide which LOC you need.

Hopefully next week I can get the 2006 3-series again and test that output for volume level, but my fear is that it is fixed like the 2005 X5. If it is fixed in level (and the volume changes happen in the amp, not the radio, like the X5), then you can do what the post describes for the X5, or you can buy a JL Clean Sweep and disable your steering wheel controls.