Hi Lanky,
what year is your A3? If it is 2007 onwards then you don't need an additiional harness, as your car would probably of had an original concert ii + or sympthony ii + which both have the same quadlock adapter.
I just retrofitted an RNS-E 2010 Model to my 2006 A3 2.0TDI 170 SPORTBACK. I needed to get the adapter harness as my original wiring was the iso leads (only 2006 and earlier models).
I got my unit of a guy of ebay who supplied me the antenna and i installed this on top of the glove box and i pick up 7-8 sats no probs (london area), so no need for a shark fin antenna as it is a waste of time as 1animal1has already stated.
Also when install is done, you need to find someone with vagcom so the unit can be coded to your car properly, especially if you have MFSW and on board computer DIS!
If your in the London area, your more than welcome to come over and i will help you install and recode as i have vagcom. It is pretty straight forward really.
Another thing also is if you have BOSE sound system, then you might have to swap a pin over in the wiring, but again this is pretty straight forward. I started up a thread on this for help when i done mine and needed help.
I will post the link when i find it.
Any other help mate, and just ask, we are all here to help out. ;-)