If you already have RNS-E and want to add AUX you will need the following parts:
OEM Aux-in 3.5mm plug (8J0 035 475)
4 pin harness that plugs into Aux-in plug (8E0 973 754)
**If you don't need the OEM look, just buy any 3.5mm plug, RCA plugs, or even an ipod dock plug, depending on what you want to connect.**
Grey 32 pin AV connector for RNS-E (1J0 972 977 G)
3 female repair wires for the 32 pins AV connector (000 979 009)
Aux In Left - pin 22 of AV Connector
Aux In Right - pin 6 of AV Connector
Aux In Ground - pin 21 of AV Connector
Recode the RNS-E to XXXXX?X to enable Aux input using VCDS (vag-com)
? = 5 to enable AUX only
? = 6 to enable AUX and SAT radio
This mod will also work with Symphony II+ and Concert II+ head units also. Just look on top of the head unit for the pin-out diagrams and connect accordingly.
On the 2009 A3's, Aux is already installed standard. If you want to upgrade to an RNS-E from a Symphony or Concert head unit, all you will need is the 32 pin A/V connector, swap 3 wires from the Aux-in. If you have Bose, an additional wire swap is need to ground the Bose.
All these parts can be sourced from your dealer for relatively cheap. No need to order from Kufatec or Vagparts (no longer an option anyways, they are closed).