What installer told you this?
I've thrown AVIC's into many BMW's and retained factory steering wheel controls.
The part you're looking for is made by PAC. The part number is SWI-PS. This part can be found all over ebay and other audio retailers that carry PAC products. The actual company is out in Santa Ana, CA last time I remembered.
You can check their website at :
http://www.pac-audio.com/