I know its a bit of a strange question but how many foglights does your A4 have on the rear..??
The reason I ask is that during some fog last week I reversed onto the drive and noticed only the offside foglight was on. Its a long time since I have had a car with just the one rear foglight so I presumed a bulb was out. I picked up the bulb and went to put it on yesterday. There is somewhere for the bulb to go but on looking closer this just seems to be a blank, and has no wiring or contacts where the bulb sits.
Now it may be that this is just a blank which is used on the continent when the left becomes the offside but I'm just curious.