I have a 1999 audi s8 with sat nav etc .at forst i though it was an RNS-D unit , but since been told , it could be RNS-C . when the satnav works , it shows in between the speedo dials as well as screen . does this mean its and RNS-d unit ?. also the unit is Square not like some of the other RNS-D units i've seen advertised. is there any other unit i could slot straight in ?