Brake Lights won't turn on 2006 BMW X5?

There could be several causes, but these are the most common:

• The fuse for the brake light circuit has failed ["blown"],
• If adjustable, the brake light switch is out of proper adjustment,
• A connector somewhere in that circuit has become disconnected,
• Corrosion has occurred on the metal connector terminals INSIDE of a connector,
• A conductor [wire] has been cut or broken, or
• The brake light switch has failed / become defective.