How to replace the left brake light bulb in the Audi A4

Replacing the bulb is very easy

Identify which bulb has stopped working because there are four of them on this side

Now, you may call this the trunk, I still call it the boot of the car, and my kids look at me and say DAAAD! 

Trunk is North American English and boot is used elsewhere in the English speaking world ☺

Anyway, inside the boot you'll find a portion adjacent to the light (as shown below) which you can pry open with your fingers or a flat blade screwdriver 

Using the right type and size screwdriver, gently loosen and remove the screw that holds the brake light assembly to the body of the car

Very gently remove the assembly by sliding it out

In the picture below, I'm just touching the bulb holder. Nothing needs to be done here 

Gently lift the white part off the grey tabs without breaking anything 

Separate the white and the grey portions by sliding them apart 

The grey portion holds the bulbs

This is the bulb that needs to be replaced

Put everything back in the reverse order