How can l replace the center / middle brake light in the back for my 2003 530i?l see a black panel at top of trunk that appears to access to the brake light - but cannot figure out how to open it? or is that the right place?

Thanks!

You access the third brakelight through the underside of the trunklid. Pry off the squarish black plastic cover, then twist off the brakelight bulb and socket.
Replace bulb. Replace bulb socket also if the bulb contact shows signs of corrosion or overheating.
Re-install removed components.