When mine gets in I'm going to get by to start with the factory amp but will be going aftermarket asap. The stock setup is pretty weak which you found out by going with the alpine.

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Just BMW's - All cars and trucks was two pages long
1994 318is
1997 528i
2000 323i
2001 X5 3.0 auto
2005 X5 3.0 auto
2011 535xi auto
2013 X5 xdrive35 Turbo
More are at my website

Aftermarket E53 Radio Install