I borrowing a little Audi (1997) at the moment. The offside back light comes on as soon as you start the engine. Is this just a faulty bit of wiring or is there a switch that I can't find to turn it off?
So if you turn the lights on the offside back light doesn't light up then, rocky.
If that is the case then it's most probably a dodgy circuit board on that light.
Just a thought, rocky. That car is knocking on a bit in age now, could be that the rubber seal between the light unit and the car body has started to perish and is allowing water / damp to get in and that could have affected the circuit board.