This part is listed by BMW as BMW part number 63256923970 (63-25-6-923-970) and is described as Genuine BMW Third Brake Light with White Lens for E53 X5 (2000-2006).
This is a Genuine BMW brake lamp with a two year warranty. Buying a Genuine part will usually be more expensive than an OEM or aftermarket brand but you are assured that it meets BMW's designs for fitment, performance, and longevity. It will also be the very latest design, incorporating any updates or revisions that an aftermarket brand typically will ignore.
BMW Fitments:
X5 Series
E53 X5 (00-06) > X5 3.0i M54
E53 X5 (00-06) > X5 4.4i M62 (99-03)
E53 X5 (00-06) > X5 4.4i N62 (04-06)
E53 X5 (00-06) > X5 4.6is M62
E53 X5 (00-06) > X5 4.8is N62
