Your going to the wrong places. Limit your search to small "mom&pop" shops. Not just any ole car audio shop, but a small one that carries high end audio.
Places like Tweeter, Best Buy, etc. have no problem finding customers, so they have no problem turning away anyone they feel (like someone with a late model Bimmer).
You need to find a shop that does custom audio for hi-end autos. I live in TN, so I would have no knowledge of audio shops in Southern Cal. We have plenty here in Nashville that can do anything you want. I constantly see late model Mercedes, Porsche, and even the occasional Ferrari role out of a shop I visit to pick up parts for my store.