I had a warning pop up on my A5 and thought I could fix it myself. I've replaced bulbs on all of my other cars. When I looked for information in the owner's manual, I was annoyed to see "take it to the dealer". That'll never do! I tried looking on-line, but nothing very helpful was available. I decided to dive in and here are the details. First, I bought the replacement lamp spec'ed in the book at the local Walmart. It called for a 921 bulb. Then, I went home and had a look around the trunk. I wasn't sure if the light was in the trunk lid, but soon found flaps on either side of the lid behind the lights. Here are what my lights look like (it's a Premium Plus w/ LED lights)
Under the trunk lid, open the flap. Carefully pull till the 2 retaining clips pop free. Once open, you'll notice 2 long nuts that are part of 2 retaining clips. Use a wrench or large flat-blade screw driver to loosen the nuts and remove the clips.
Once the clips are removed, unplug the electrical connector and you'll be able to lift out the light assembly.
The reverse light is located in the corner, turn it slightly counter-clockwise.
Now, you can replace the bulb.
As they say, reassemble in reverse order!
Any questions, please ask!