Are you talking about the black trim on the exterior, like running boards, rear black valence around exhaust, etc?

If so, the proper way to do it is Meguires back to black (can get it at walmart like 5-6$) or Stoners Trim Shine, (can get it at most autoparts stores) i prefer the stoners, used to use back to black and have it in my detailing kit if needed...

And the other way to do it is after you wash/dry/wax whatever you do your car, just run over it with tire shine, make sure if you get any on to paint to wipe it off.

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2006 X5 4.8is Fully Loaded