Parts:
An RNS-E for an A6.
Sat Nav DVD from Audi or e-Bay (2007 or later for TMC)
Adapters: http://www.kufatec.de/shop/ part numbers 34521 and 33406
-Please check with Kufatec if you are unsure what you are ordering!
-This kit includes the radio removal tools.
-RS6s with the RNS-D Don't have the small BNS

You will need to join a couple of wires for the CAN connections so will need tools for this.
Some useful references:
http://www.audiforum.us/
http://home.comcast.net/~nelson.paiva (PDF Install of RNS-E into US RS6)
http://www.nsxjr.com/downloads.html (PDF Install of RNS-E into US A6 Allroad - very detailed)
The above guides are useful but they relate to replacing the Symphony 2 unit rather than the RNS-D.
Tips:
1. This install is easy if you get the right bits. I've heard bad things about some of the eBay adapters.
2. On the Kufatec adapter above the GALA wire was already wired in so you don't have to worry about it.
3. You will need to wire the RNS-E into the CAN bus behind the cluster, grey block (T32c) pins 5 (CAN hi) and 6 (CAN low). The Kufatec adapter has 2 wires that just need to be connected.
4. You will need to check the coding of the Radio with VCDS or similar post install.
5. It will take a while to find the satellites and sort itself out. The first local drive I went on the Sat Nav was all over the place. The next day I went for a longer drive and it was fine. I checked the satellites and GALA signal in engineering mode and all was ok.
6. There is very little space behind the unit itself to fit the adapters - there is space behind the AC unit if you can squeeze the blocks down into it.
7. Don't press on the LCD screen when pushing the unit in - yes I know it is obvious but some people forget!
8. To get TMC you need a later DVD (e.g. 2007 or later if I remember correctly!)
9. My arm rest cradle phone (Nokia) didn't work properly until I updated the firmware with the later DVD (Thanks for the tip PetrolDave!). This is done automatically when you insert a newer DVD for the first time.
Engineering Mode.
1. Turn on RNS-E.
2. Press the NAV button and release.
3. Press the top-left cursor button for 5 seconds and release.
4. Press the top-right cursor button for 5 seconds. Hold until the screen changes.
Note, sometimes this didn't work - usually because I was already on the NAV mode when I pressed the NAV button. I pressed "Radio" and tried again from 2 and it was fine.
*disclaimer - this information is provided as is, in that this is how I installed my RNS-E. If it doesn't work, tough.
