BMW Questions? Ask a BMW Repair Expert for Answers.
To clarify, do you mean the 3rd brake light? It's right in the middle above the back window on the hatch?
If it's stuck on, there may be a short in the light, but first we need to check if the other two brake lights work. Have someone press the brake while you check whether the other two brake lights come on. If they don't, you'll need to pull and check those bulbs. It's possible all of the brake lights are stuck on (which probably means it needs a new bake pedal switch) and the third brake light is the last one that hasn't burned out yet after being stuck on for too long.
If the other brake lights work just fine then there is probably a short in the third brake light that will need to be traced by a qualified mechanic with a good multimeter. I can send you a picture showing how to disconnect it if you want to shut it off in the meantime.
I just wanted to check in and see if you needed any additional information on this question? I haven't yet seen a response from you and was hoping to help in any way I can.