Just done mine on my S3, not that scary a job if you take your time, there is loads of info on the A3 board if you have time to have a look. biggest job on an A3/S3 is swapping the switch in the arm rest and replacing the plastic welded fixings.
I am sure that there is someone with an A4 who has done this but if I can help, shout.
(should this really be on the A4 board?)