How do I replace the rear light on Audi A3?

Its not to hard to do! Inside the boot behind the light cluster there is a blank plug that you need to pop out about the size of a 50p (large coin). In there you'll see a long plastic hex nut with a slot in it. It easier to use the screw driver in your spare wheel kit (probably designed for the job!). Unscrew this nut BE CAREFUL not to drop it in inside! You then need to hold the rear cluster by the inside edge (boot/hatch open) and pull it out a bit to clear the thread you have just removed the plastic nut from. Move the front of the cluster sidewards which will pop the side of the light out of the side part of the bodywork. Its only held in by 2 plug style things in rubber gromets. Remove the black plastic covered nuts holding the grey plastic part and separate the grey from the red. The rest is simple, the bulbs are held on a circuit board like any other car in the back of the cluster. I would use Phillips bulbs though, I put some cheap petrol station ones in and it kept blowing them! Hope this helps!