If you have the A4 version of the RNS-E (8E0 xxx xxx x) that gets it's speed signal from the CAN bus.
It also gets the reverse light, illumination and ignition signals via the CAN.
I did the install in my 2004 B6 S4 last week using a plug & play harness from Ralf and antenna adapters from Kufatec - it replaced a Symphony 2. Took about an hour to do the swap, and a further hour to get the RNS-E and instrument coding right using VAG COM.