I found the instructions in my manual under "lamps and bulbs" (page 144 in my manual but its probably different in yours since I have a 328Ci). Its a bit involved so its best to look at the manual then for me to relay the info, plus there are illustrations in the manual. The dealer will replace them under warranty.
you have to open the trunk and you know those gray circles? you have to pull them out w/ a flat head screwdriver. once you get those 4 top circles out you can pull the liner down and see that behind the reverse panel is a nut that you have to remove with a socket. after that, the panel is pushed out just like that.
The 'grey circles' are 2-piece expansion plugs. You have to pry up the center pin with a screwdriver, and then the pin and expansion plug pop out in 1 piece (you can't separate the 2 pieces).
You also do not have to remove the entire liner from the trunk lid - it is very flexible and will flex enough to give you access to the reverse light once 2-3 of the plugs are removed.
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