Well you'd have to check the contacts on the board of the LED's, I'm actually more inclined to think its a bad connection at the connector.

If I recall correctly the painted piece on top of the trunk spoiler pops off with clips, and you can unplug and remove the 3rd brake light once you have the top trim piece off.

I have a white 3rd brake light on my car, which I believe was standard with Sport package. However, in 2006 and on the 4.8iS (correct me if I'm wrong) the 3rd brake light is red.

Either way they are ridiculously overpriced at about $250 from the dealer. I'd get one off a junked X5 before spending price on a new product.

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards