Arizona AutoHaus, Pelican Parts, Bav Auto, and more.  I did a retro for roll hoops on my 97 Z3 2.8 but I got a kit from the dealer for less than $600.00.  You have to check your build date, anything built before 1/97 is not able to be retro fitted.  According to RealOEM the kits for retro fitting are: without Harmon Kardon subwoffer 54619408817 $669.60 ($750.00 at Pelican parts), and with Harmon Kardon 54619415342 $591.48 ($530.00 at Pelican Parts).  Your dealer is ripping you off!
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