Soundgate makes a switcher called the AUX3 that is also available with a BMW changer interrupt harness.

You insert this switcher between the changer and the car harness, and use a three-position switch (an attractive black one) to select which input to use.

If you need a line driver to step up iPod level I know Soundgate makes one that is cheaper than the Overdrive and intended for fixed-level applications (the Overdrive is intended to work with a wide range of input voltages such as come off of a head unit with a volume knob,and is thus more expensive to make). You may or may not need the line driver.

https://www.soundgate.com//index.php...&usereferrer=1

https://www.soundgate.com//index.php...&usereferrer=1

We sell these for like $75 each. The only kink is that the location of the switcher and preamp can make the device pick up noise in the car, so you have to move the gear around a little to determine where it should be. (Some devices in the CD Changer areaemit EMI that can be picked up by audio devices).