If is is only coming on when it is in reverse then it has to be tied to the reverse lights somehow. On the standard 7 pin plug, typically the center pin is the reverse lights. Here is a link to the wiring diagram for one
. I would consider putting the camera on the running light circuit, you don't want it to have power all the time, eventually it will "burn" out. As far as where it is hooked in... good luck, I would start at the camera end and check the wire color then look in the wiring bundle with the rest of the lights near the front.