Thanks to everyone who posted possible causes of non functioning CC on 1999 A4 1.8T Q. I am a fairly experienced mechanic but had been unable to find the problem. After about 3 hours of chasing it down this morning I found fuse 11 was loose. I had previously checked that and it was OK but my 6th sense told me it was not firm enough in it's holder. I spread it's little legs and voila, I have CC again.