If you have that musty odor, check first to make sure the a/c collection can is draining properly - once in a/c season, look for the regular drip of evaporated water dripping from under the vehicle. You can then feed plastic tubing [in my old E28 days, a wire coat hanger!] up the drain hole to make sure its clear. I expect there is a DIY for killing the bacteria in that collection tin, but I haven't seen it on the E53 forum .... Once you are well into a/c season, provided its draining, the musty odor should go away .....

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2006 X5 3.0 6-spd w/Evo UUC ssk
sport/premium pkgs
born Valentine's Day, 2006.