Ok, what you are referring to is the switchbox for cars without TV fitted. I bought kit 66 21 0 303 079 but this is for cars with Nav and TV, you are going to want kit 66 21 0 447 960 as per this link
RealOEM.com BMW E46 M3 Retrofit backup camera

I have uploaded the EBA retrofit guide here:
RapidShare: Easy Filehosting

As I bought the 079 kit it does not list the switchbox part number in the parts list therefore I can't tell you what it is. However my parts list contains part numbers that don't look like the usual BMW part numbers and I did a search by switchbox on RealOEM and nothing came up so I am gussing that you can't buy the individual components.

Good luck with it.