Question: back up camera on aftermarket radio

Hey guys I'm getting a back up camera installed in my truck tomorrow. I ordered the camera off eBay and got it today. The guy who's doing it for me said that I couldn't use an oem camera with it so I bought an aftermarket one. The head unit is a kenwood dnx680hd. He said its camera able but I don't understand why he has to rewrite the hole set up when the truck comes pre wired through the tailgate... Does the wiring go past the tailgate or does he have to wire all the way to the cab himself.
Thanks

 

Bump

 

A BOYO license plate frame w/camera is a good option, I've installed them in a few vehicles.

As far as the truck being prewired, search the audio/video forum and you'll probably find a answer.

 

There's a lot of useful information in this thread and this thread.