The answer is clearly stated in the retrofit instructions:

C = Supplementary wiring harness for navigation systems in cars with a production date prior to 9/98

D = Supplementary wiring harness for navigation systems in cars with a production date prior after 9/98

And you can always make this harness yourself, it only has the speed pulse and reverse signal (both of which you can tap into in the boot)

The fact that you are asking questions about topics which are plainly stated in the retrofit instructions leads me to believe that maybe you do not have a copy of the retrofit instructions. If you don't have a copy, you should get one, because the instructions instruct you very clearly on what to do.