Remove the plastic cover under the front of your X5. I believe all of the bolts are 8mm. Then you should see a blue drain plug on the radiator (driver's side of the X5). You'll need to remove the radiator cap as well.

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2006 X5 3.0i: Jet Black with Truffle Brown Interior, Auto, Navi, Xenon, Premium
2007 335xi: Black Sapphire Metallic with Black Interior, Manual, Xenon, Premium, Sport, Cold Weather