I had my 02 X5 outfitted with a JL Audio 250.1 Amp and a JL Audio 10" sub. They did a really clean install and mounted the amp in the driver side cargo area behind the cover.

They are a bit pricey but they do clean installs.

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Previous: 2002 X5 3.0 / 121,000 miles
Current: 2007 X5 3.0si / 93,000 miles