is your '03 S8 a D2 or D3??.. you can not install an RNS-E in the D3s they use the MMI system from Harman/Becker and utilize MOST
if your dash looks like this
you must have a D2 and i believe the RNS-E will work in your car..
there is not a direct replacement, but i assume you would use the A6 version and modify the faceplate a bit..
you may want to check with craigby, looks like he has done it before..
http://www.navplus.us/forum/viewtopic.php?highlight=s8&t=74
good luck..
if your dash looks like this

you must have a D2 and i believe the RNS-E will work in your car..
there is not a direct replacement, but i assume you would use the A6 version and modify the faceplate a bit..
you may want to check with craigby, looks like he has done it before..
http://www.navplus.us/forum/viewtopic.php?highlight=s8&t=74
good luck..