Really depends on the head unit you got! I've got the kenwood dnx520vbt and am in the process of fitting a basic cheap pal/NTSC camera (about £30) sensors isn't really an option on this unit. If you have an rns510 then you have more options
Let me know how you get on with this, I've got the same head unit and wouldn't mind seeing how it turns out.
I put a camera in my mates van, he had the same HU as you before getting the 921DAB - just the same as a camera install on any other aftermarket for VW stereo really.
Install any old PAL/NTSC camera, run the video cable to HU and either power the camera from the HU or reversing lights (also tap into number plate lights if you have a number plate light type), set it up in the menu and that's it.