Back your X up to a wall or building at night. Look in your mirror as you press on the brakes. Should see a reflection of each working light off the wall.

You have one bulb in each corner. Not sure how many are in the center high mounted brake light.

You can get generic bulbs from the parts store (at least I did anyway). They are pretty easy to replace but you need small hands to get in there, twist the bulb holder to release it and then pull it out (there's a hatch by each corner brake light -- look at it closely and you'll see the release). I managed to drop the bulb holder pulling it out and added a few minutes figuring out where it went.

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Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual