Had a quick google search and found this link have a full read of the article to make sure its for the a4 b7 which im assuming u have
http://www.tyresmoke.net/forum/topic/60296-how-to-retrofit-a4-centre-armrest/
Have a read if u feel confidant in doing urself have a go. My mate fitted armrest in his a3 himself he said it was fidly and had to cut a hole into the area requires lot of patiance and time cus u dontbwanna cut too much and arm rest is loose as i hear a badly fitted armrest could potentioly be a mot fail
Hope the link helps if its not for the b7 then im sure u can find more articles on net