a NavPlus unit in my 99.5 A4. right now it's the non-bose system with a single din dash. I haven't found a write up for it, just seeing if it's possible.

With a digital clock you have at least a CAN bus the RNS-E will talk to. In this case you would only need to convert to double DIN and deal with different time on your cluster and the RNS-E.

Without a digital clock there is no CAN in your car and you will loose for sure radio/navi display on your DIS. You would need to get a CAN adapter that makes a CAN signal out of your ignition/light/speed signal.

In either case you would have to deal with faceplate color differences and only 95% perfect fitment.

Search navplus.us for B5 RS4 or S4 projects, there are a couple. Without CAN and in a B5 I would probably go aftermarket instead of OEM.