Seems to be plenty of options out there. The obvious answer is search but if you want get a less than detailed answer, here it goes...

crutchfield shows options-pioneer, alpine, etc, eonon seems to be more than a half decent option if you get the latest, avant sounds like the best but expensive option.

I stuck with oem and added bluetooth by aux cable to cd player-nice!

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2003 530iM
2001 X5 3.0