brake light not working on one side

My brake light on the drivers side started to stop working now and again and is now off all the time. I changed the bulb and that didn't help, in fact when I looked at it again it had turned black. I thought it may be an earth problem but the wiring diagram just says that the earth is in the rear wiring loom so I don't know where to check.
 
Any ideas most welcome.
 
Thanks,
A4 avant 1.9tdi 1998

thrs two buttons 1 for fog lights and another for fog lights.
 
in german when its really foggy you turn both on, so a car can tell your going slow. so 1 light turns off.
 
check your fog lights if thr on.

The brake light seems to work fine when the main lights are turned on? Suggests to me the bulb is Ok as is the fuse and earth? Not sure really what to look at next - please help if you can!!