Last I knew, BMW required dealers to provide proof of ownership before they would make a key. I know there are vendors that make some BMW keys but I don't know if they can make an E53 key. Recently, I costed out the getting a 335 key with convenience feature. Dealer price was over $500.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas